The District Line had a particularly complicated set of destination options at the time it was electrified in 1905. The fact the line was substantially underground presented some interesting problems for signalmen, often quite unable to see approaching trains or their destination-specific headcodes. To make life easier, a means of transmitting codes ahead of trains was devised, essentially using what would now be described as a four digit binary coding system coupled with temporary storage devices. This presented signalmen with data about the next train (or trains) approaching.

Once the decision had been made to introduce such a system, it became obvious that by tapping the four wires through which the codes were transmitted at each station, so it would be possible to display the destinations of up to three approaching trains. To achieve this, large metal signs were erected displaying up to fifteen possible destinations; each destination was presented on an enamelled iron plate and obscured glass arrows next to the relevant plate were back-illuminated when the appropriate code was decoded by the local 'combinator' (as the binary to hexadecimal converter was called).

The District still uses the 4-element binary coding system to transmit train descriptions for signalling purposes, but for the last couple of decades modern dot matrix train indicators have been used, each operated by computers.

As far as I have been able to tell, the displays shown on these indicators has not properly been recorded, nor the varying destinations shown over the 80 years during which the indicators were in use.