1. C-APT
2. InterCity 225 testing system

During the 1970s, with the introduction of the Advanced Passenger Train - Prototype, a new signalling system had to be developed in order to allow the units to exceed the national maximum permitted speed of 125mph, and reach up to 140mph, as previously promised during funding allocation presentations.

This took the form of markers placed on the track at specific intervals of 1km or less, which would store the maximum available speed limit. These could be changed in real time based on weather or lineside events, which made safe running at 140mph possible. C-APT stands for Control - Advanced Passenger Train.

The APT units themselves contained transmitters, which would send a continuous beam down to the track. Upon making contact with a C-APT plate, the transponder of said plate would send a return data stream containing the maximum permitted speed to the receiver contained within the APT unit.

This was processed by the onboard computer, and would be displayed on a small screen within the activated cab of the unit.

Another use of the C-APT system was to detect the proximity of a tunnel and forcibly close air intakes to avoid a sudden change in pressure that could cause discomfort for passengers.

Due to the speed of operation undertaken by the units, each contained several failsafe mechanisms. For example, each unit contained two sets of computer equipment in case one set failed.

Furthermore, the computer was programmed with the interval between markers, so that if 1km passed without detecting a plate, an audible warning would sound in the cab, which the driver would have to acknowledge before proceeding to decrease speed to the regular maximum speed of 125mph.

During the testing of the class 91 locomotives and Mk4 coaches and DVTs, the opportunity was taken to modify the IC125 system by adding a single new marker, consisting of a flashing green light.

This would work in conjunction with persistent green, however flashing green meant that line speed could be reached safely as there were no trains ahead within a suitable distance.

The system was officially taken out of operation, however the signal modifications were never undone, so these signals can still be seen today, along the ECML around Peterborough. Whilst it serves no official purpose, drivers enjoy the enhanced informative possibilities of the system.