Mind the Gap

Why there are gaps

There are many reasons why Underground lines are not straight lines. It may be that the best route to serve passengers is not a straight one. It may be because the older lines felt constrained to run directly beneath roads to avoid the expensive complications of buying up property or negotiating wayleave rights, once more difficult than now. The fact is that the older lines twist and turn. We users are often unconscious of this, unless one of the turns happens to be at a station. Then we get the problem of ‘gaps’.

Gaps between the platform edge and the railway car occur where the platform edge follows a curved path. A railway car (the Underground has ‘cars’ rather than ‘carriages’, following American practice), for fairly obvious reasons, has straight sides. It therefore follows that if one part of the car is right next to the curved platform, then other parts cannot be. The problems arise at doorways. These tend to be spaced evenly along a train, so at a curved platform there will inevitably be some doors that are right next to the platform edge — the ideal position — but that means some other doors along the train will be an unnaturally long way from the edge, with the possibility that someone not looking, or losing their balance, could fall into the gap. Hence the warning — ‘mind the gap’!

Now, since we know where the gaps are, and the doorway positions are selected by people and are not a mere accident, there might be those among you who would think that the gaps at doorways should have been done away with by now, but it is not so simple. For a start there are two sorts of gap. Where platforms are situated on the outside of the curve, the railway car ends are located as close as is possible to the platform edge, but that means the centre of the car will be some way away. On the other hand, where a platform edge runs along the inside of the curve, it will be the centre of the car that is right up against the platform but the ends will be a long way away. The same cars have to cope with both situations, so the outcome is that compromises are made.

It is a tad more complicated than this because tube-type cars are so small that the tops of the wheels are above floor level (you can’t see them because they are always boxed in by seats), and that means that until new trains arrive with small wheels the doors must be either side of the wheel locations, rather restricting where they can go. On tube cars this usually means large doorways about one third and two thirds the way along, and smaller doorways at the ends in the small gap between the wheels and car ends. Trains didn’t always have doors here but that left very large intervals between successive doors which caused congestion, so from 1930 end doors have always been provided. The outcome is that at curved platforms the main doors will always present a gap (but never the maximum gap between platform and train), but the end doors will either be the closest to the platform or the furthest away depending on type of platform.  The gaps, by the way, can be up to about 400mm in the worst cases.

Gap with platform outside

Gap with platform outside

The upper photo shows an 'outside' platform leaving gap in centre of car, while the lower photo shows an 'inside' platform leaving gaps at car ends. Visible in both photos are the 'Mind the Gap' warnings painted onto the platforms at doorway positions and the shelf under the platform edge (with the black and white stripes) that would help break the fall of anyone stepping into the gap.

Mitigating the risks

If we cannot eliminate gaps, you can be sure that for about a century railway staff have been struggling to mitigate the risks they present. We must first turn our minds back to the kind of railway that existed in those days. Tube trains (the little ones) comprised rakes of cars that only had doors in the ends. These led onto small, open, gated gangways. Each gangway was manned by a ‘gateman’ who only opened the platform gates after the train had stopped and could warn passengers about any gaps and even help them on and off. This probably contributed to a climate where curved platforms were tolerated with less concern than we would expect today.

It so happened that because the entrances were at the extreme ends of cars, the only problem stations were those where ‘inside’ curves existed — these are the ones which maximize the gaps at the ends. These stations were Wood Lane and Bank (westbound platform), and Paddington (northbound platform); frankly with only these stations and lots of staff, the problem could be managed. At other curved platforms, the entrances were, if anything, brought closer.

The next stage of development saw the slow introduction of doors in the centre of cars to meet demand. The first line to have these was the Piccadilly Line, from 1914. It is a curious thing but virtually every platform is straight, more or less. Piccadilly Circus has kinks in it and there is a nasty curve at the south end of the eastbound platform at Holborn (an outside curve), but this only extends over about two car-lengths and as many trains were quite short the ill effects could be avoided altogether.

It wasn’t until 1920 or so when centre-doors appeared on the Bakerloo Line and this highlighted the unfortunate gap problems at Paddington and Waterloo. The risk was understood immediately and the solution was very simple — the centre doors were locked out of use at the sharply curved platforms. This achieved the objective, but was counter-productive in that it was at these very stations they were most needed and it must have been confusing for passengers and appeared perverse.

The Underground’s goal was the introduction of automatically controlled doors at frequent intervals along whole trains, and for all trains to be equipped. This would improve general safety, reduce boarding times and allow more trains to run to increase line capacity. It would also reduce staff numbers and costs (which were always very tight). Safety was increased because doors would always be controlled by (ultimately) one person, would be positively locked shut, and be proved shut before the starting signal could be given. The controlling medium was compressed air (the Underground uses compressed air in unimaginable quantities), so trains fitted with these doors were called air-door trains.

Air-doored trains were introduced in large quantity in the 1920s. These generally had wide twin doorways immediately inboard of the bogies (the structures carrying the wheels), thus producing a modest gap, whether alongside either inside or outside curves. Exact operating instructions have not been determined, excepting that on the more sharply curved platforms the edges were painted white (it was not then generally the custom to white-line edges) and certainly by 1926 station and train staff were required to call out ‘mind the gap’ at stations where large gaps existed. Accidents where the gaps were implicated had to be specially reported upon, noting exact point where it happened.

Waterloo 1923 showing edge lighting

This pictures shows Waterloo in 1923. Platform under-edge lighting has already been installed, and the white-painted under-edge shelf is in position, but no warning notices are evident.

By 1928, it was no longer (apparently) necessary to paint the edges of curved platforms, but instructions draw attention to lighting having been provided beneath the edges of curved platforms at tunnel stations, together with ledges (which had to be painted white). Train and station staff had still to call out a warning. This may well have resulted from a staff suggestion made in March 1922 by Gateman T Jones of Hammersmith depot — apparently implemented at the end 1922 after which centre doors on the rear car of a new 6-car train with automatic doors could be used at Holborn (eastbound) to deal with the gap at its south end.

We also know that from May 1934, the under-edge lighting at Charing Cross (Northern Line northbound) and Waterloo (Bakerloo northbound) was altered to illuminate only when a train was berthed in the platform. This was achieved by means of a projector lamp, mirror and photocell arrangement, the lighting being arranged to switch on when the beams (incident and reflected) were broken by a train correctly berthed. Piccadilly Circus followed in February 1935, and Waterloo (Bakerloo southbound) in March.

Waterloo showing gap signange

A photograph dated 17 January 1936 shows signs mounted at Waterloo along the platform edge at train height level lettered “Beware of the gap” with “Mind the gap” markings on platforms at intervals along the edge. This may have been installed with the lamp switching or later during the year. A 1951 description of the gap problem suggests these overhead signs flashed (and I dimly recall them doing so). This reference also refers to the earlier need for staff to ‘bawl out warnings’ but rather implies this was no longer done, or at least not reliably.

I should break off here to say that pretty much everything I have just said applies more particularly to the small tube-type trains. Those trains on what later became the District and Metropolitan Lines always had hand-operated doors opened, and, when cold, closed by the passengers and there were always several staff on the train with much platform staffing. The aim was for straight platforms but many were curved and presented some very large gaps too. Trains with air door control were introduced in 1936 and most of the fleet was converted by the mid 1950s.

1951 instructions note that by then all platforms had painted white lines along the edge except those on curves where the under-coping ledge was white instead. Between 1951 and 1964 arrangements were again changed with stations with gaps now having a white line along the edge (like everywhere else) and the under coping ledge being painted with black and white diagonal stripes (as today).

We may therefore take it that the expression ‘Mind the Gap’ can certainly be dated to the early 1920s, though it is very likely that it goes back farther than that and quite possibly to the opening of the tube lines having the gaps (perhaps 1900 in the case of the Central London Railway). Audible warnings were given by staff, and we can see that notices and warning signs were provided. That is the position in the early 1960s, but what happened then?

Recorded Announcements

The short answer is that the only new measure developed to deal with curvature-induced gaps was the slow roll out of automated announcements. This may sound particularly odd to anyone who is aware of the huge staff shortages in the 1960s and the difficulty in providing adequate staff at busy stations with curved platforms.  The reality was that there was not only no suitable technology to do so, but at platform level there was no technology at all, at least, not beyond the station lighting and the odd telephone. The first public address systems didn’t arrive until 1944, and that initiative was confined to just twelve busy stations. A few more were slowly added over the years, but even in the early 1980s a large number of stations had nothing better than a megaphone. It is true that Holborn was equipped with a CCTV system and centralized public address in 1962 But that was purely experimental and its fruits were not realized until the Victoria Line opened in 1968. After that the system was very slow to catch up and it wasn’t until after the fateful King’s Cross fire that modern communications became universal. Without public address there could be no recorded announcements either.

There have been recorded announcements on the Underground for many years, but only covering very small areas. More than one attempt has been made to encourage considerate behaviour on escalators; an attempt was made, for example, in 1920 using a kind of compressed air amplifier that took a signal from a gramophone recording to bawl out 'Stand on the Right'. From the 1930s announcements were to be found in lifts; this one went ‘Stand Clear of the Gates’ followed by an audible click. The technology here relied on the use of a film strip and photocell arrangement in the same manner found on film sound tracks. The click was where the word ‘Please’ had been excised, presumably by just painting over the relevant part of the sound track. The offending word had evidently been the subject of debate and those who felt that the feeling of urgency should over-ride that of politeness finally won the day. It is regrettable perhaps, but having spent many years on the Underground it is a feeling I would have to agree with: if crowd-handling matters are not treated with urgency they can all too easily get out of hand. I remember the thin voice used in the lifts, but they were only used in the original automatic lifts of 1930s-50s period because the doors were closed by a timer rather than a person (most lifts were staffed).

Such primitive technology was not felt suitable for use on platform public address systems, for some reason. I now realize that not very much has been said about station public address systems and automatic announcement systems. It is a matter I need to remedy, but at this moment do not have much to hand, so will forebear except to mention that the pundits think that London Transport went for a solid state system in around 1970 and these were deployed at odd locations thereafter until the network was modernized at the end of the 1980s. The word digital has been used, but this date seems to me far too early for that. More anon.

More than one sort of gap

I mentioned earlier the words curvature induced gaps because those gaps are specifically related to curved platforms. It should be perfectly obvious, though, that there are other kinds of gap. For example, even along straight platforms there must be a gap between train and platform, for otherwise the train would scrape along the platform edge, which is not a good thing at all. How much gap? Well there are modern standards that are designed to ensure gaps are as small as practicable but much of the system is very old and built long before there was any standard at all. It is not helpful that the width of trains can vary and that different station owners in the past have each had their own practice. The various owners of the Underground have sought to avoid flamboyance whenever tempted to do so and settle on fairly boring train designs that, at floor level, maintain a degree of standardization. The gap risk is mitigated by the use of floors that are higher than the platforms and use of sill plates at doorways that ‘oversail’ the edge of the platform (on some trains the train sides over-sailed slightly as well. This had the disadvantage of creating a step, but a compensating advantage that in a directly vertical plane there was no practical gap at all. It was a feature that worked reasonably well.

I should remind regular readers of my blog item on train floor heights. In brief, the good Mr Yerkes, who built and modernized much of the Underground in early Edwardian days, wanted train floors that were level with the platforms. He felt a step hugely reduced boarding and alighting times at stations. Unfortunately, for technical reasons, train floors could not then be lowered so he wanted to raise platform heights instead. This led to lengthy and fruitless discussions with the Board of Trade, which was then responsible for railway safety, and who were not at all helpful. Thus for another century we have had a step at the platform edge.

Now the new Underground S stock has got lower floors which by and large are at the same level as the platforms. The only way this can be achieved is by pinching in the width of the train at floor level, for, without over-sailing, the floor must be narrower to provide clearance. I must see if I still have some boarding time data for the old trains and see if the same-level entrances are any quicker, but it was mainly done to benefit those less abled people who found the step a problem. Of course, this now means there is an all-too-obvious gap at every platform, but it is fortunately not usually a large one. It also means a few already large gaps on curves are now even larger, possibly by 50mm or more.

You will have perceived from what I have just said first that there must be horizontal gaps at any station, it is merely that they are not usually very big and assistance can be given by coloured lines and floor level train lighting, which is now standard on new trains. Secondly I have introduced the matter of vertical gaps, or steps.

More Compromising

Vertical gaps will be partly managed away as new trains arrive, but the Underground has what may be a unique historical problem to cope with, and it will be very interesting to see how things pan out. I have mentioned tube trains (the small ones) that are designed to operate in circular deep level tube tunnels. There are then the big trains that operate on the Metropolitan, Hammersmith & Circle and District Lines which are more or less the same size as main line trains. At doorways, the floor height of a tube train is about 685mm above rail level while that of a big train is about 1125mm.  All was well when each train type kept to its own system, but inter-running of services became a fad for a number of years and the present system is stuck with them – most are rather useful so one cannot just stop inter-running.

Inter-running occurs between Rayners Lane and Uxbridge, Acton Town and Ealing Common, and over the London Overground between Queens Park and Harrow & Wealdstone. The problem is what height to make the platforms. If they were either of tube height or of main line height then the resulting step up or down would be excessive for even able-bodied people, and a big step down into a tube train also risks the banging of the head on the doorway roof. The answer, by no means entirely satisfactory, is to share the pain by making the platform an intermediate height, usually, and very appropriately, called ‘compromise height’. This is about 840mm above rail level and suits nobody, but at least the able bodied can prepare for it and cope, by and large. I have seen people struggle. Of course this gives the tube trains a problem because if the standard arrangement is to over-sail platforms then the sills will hit a higher level platform. In consequence the platform nosings must be set back more, but I wonder if under-sailing the edges has been tried out. The use of S stock has reduced the step up at compromise stations, but still leaves a big step for the tube trains. Of course, if lower-floored tube trains are introduced as planned then it will amplify this step and be awkward to manage during any stock changeover. Dual height train operation used to be even more common than just described and has left traces all over the network where there are now random height steps up and down (eg on Northern, Central, Jubilee and Piccadilly Lines). These are slowly being managed away, often by altering track levels, but it makes the concept of a standard platform height an interesting one.

We see, therefore, that there are horizontal gaps, vertical gaps and there will obviously be combinations of the two. The wide gaps are an annoyance when it is busy and you can’t always quite see them when it is very crowded, especially when getting off. There are not really many really nasty gaps on the Underground, but on the shared section up to Harrow I would commend the rear end of a Bakerloo train at Willesden Junction for a nasty example of huge step height difference combined with a curve-type gap.

Willesden Junction

This one, at Willesden Junction is about 15 inches.

Lest anyone think that the Underground is uniquely blessed with gaps I must put them right and draw attention to some really evil gaps on National Rail (and of course on many continental systems there are no platforms deserving of the word and steps must nearly always be used). For those you would like a memorable experience I commend platform 17 at Clapham Junction when it is busy (which it now is much of the time). Now that is a gap: one really needs to plan one’s exit from the carriage before testing it. I nearly visited its inner workings myself when some maniac right in front alighted and then stopped dead, obvious of the fact they now constituted a serious obstruction and leaving me in mid leap, cartoon like, and with nowhere to land. I reckon that gap is over 500mm and it combines with serious difference in platform-train height.

Level Boarding Areas

One means of mitigating gaps, particularly with disabled customers and wheelchairs in mind, is to provide a section of platform at each station that is the same height as the train floor. This is only useful if the elevated sections line up with the same cars at every station and if platforms are consistent in height so as to enable a useful number of stations to be fitted.

The Victoria Line is equipped this way. However the new trains still have a door sill that oversails the platform edge slightly so simply raising the platform would obstruct the sweep of the sill. The solution adopted has the edge of the raised section set back a little so the sill can pass by yet leave only a small gap afterwards. The photo below shows the arrangement.

Victoria Line level platform

At bottom of photo a door sill is just visible a few centimetres higher than ordinary part of the platform and slightly overhanging. Ahead is a raised portion of platform, ramped at each end. The raised edge is set back, eliminating vertical gap but introducing a small horizontal gap instead. The original platform edge is just visible underneath. I'm not sure the slightly unsettling shifting of the yellow safety line and tactile surface is what the standard-setter intended and suspect a better approach to this visual arrangement might be called for. The raised section is about a car's length

Platform supervision

The mitigating measures cannot eliminate all risk and so accidents do occasionally happen and someone will fall into a gap. This is not frequent given the volume of passengers and there can be many reasons for it happening, not least alcohol related activity. The actual reasons are not particularly relevant to this theme; what we must consider is what happens when someone has not minded the gap. On the whole, reliance is placed on the train operator checking that it is safe to proceed before taking the train away. Modern trains have in-cab CCTV that relays pictures from multiple cameras that are set up so they can see the platform-train gap along the whole length of the platform even if crowded. Operators of older trains achieve the same ends using multiple CCTV monitors on the platform, opposite the driving position. Resolution is pretty good and virtually all incidents are spotted before a train moves.

At busy or awkward platforms staff are available to see trains away. The platform staff member is required to raise a croquet racquet (well, that is what it looks like) when satisfied it is safe for the train to move, and to keep it raised until the first few cars have departed. The operator is required to check the racquet can be seen as the train pulls out and stop and investigate immediately if it disappears. Managing the platform-train interface (of which the gap itself is only one element) is a real problem, but it must be done as it is where a large proportion of accidends do happen. 

For the future the gaps should slowly be managed away when reconstruction schemes emerge, but recent evidence suggests this could take a century or longer to do as it is logistically very difficult and the costs rarely warrant it. Possibly moving ramps or something of the sort, worked in conjunction with platform edge doors, but this, too, would not be cheap and reduces flexibility if it is ever desired to change trains.

I have so far kept my observations fairly general, but I have spent a little time examining the various gaps, at least at tube-type stations. I have also made some approximate measurements, which I offer in the table below. I have tried to get measurements correct to about +/- 25mm. It is a surprisingly tricky business as the door sills are invariably much higher than the platform and it depends where along the sill one tries to measure. I would suggest they are good enough to indicate the general ordering of gaps from very wide to very small, but no more than that. The worst gap so far identified is Bank, eastbound Central, for about three car lengths at the west end where a number of double-doorways present a very bad gap. Piccadilly Circus, Bakerloo, has several quite nasty gaps, but only over very short lengths that do not affect very many doorways. Bank, Central Line, westbound is the next worst and affects the whole platform, though only end doors.

Station Line Direction Location Approx Gap mm
Bank Central EB West end 375
Piccadilly Circus Bakerloo SB North end 350
Piccadilly Circus Bakerloo NB North end 325
Piccadilly Circus Bakerloo SB South end 325
Bank Central WB All 300
Waterloo Bakerloo SB North end 250
Mansion House District EB West end* 250
Paddington Bakerloo NB North end 225
Waterloo Bakerloo SB South end 225
Embankment Northern NB Middle 200
Paddington Bakerloo SB South end 175
Waterloo Bakerloo NB South end 150
Waterloo Bakerloo SB Middle 150
Paddington Bakerloo SB North end 125
Piccadilly Circus Bakerloo NB South end 100

* D stock only (owing to central door position)

Anyway, that’s what I think there is to say about gaps. Now can we please stop going on about ‘Mind the Gap’ and move on!


The Drawing Below

The historic drawing below shows typical platform arrangements with on the left a tube train door sill in its relation to the platform edge (tube and compromise height platforms), and on the right the surface stock equivalents. The over-sailing arrangements and step heights are clear. The extreme left example appears to suggest that gaps should not exceed 12 inches, but must emphasize that this represents only the ideal. So far, I have only found four locations that in practice exceed the old standard, but there are almost certainly more.

Plan showing standard platform heights