[SDX]

Hello, and "bon courage" for your project!

The current track-circuit systems vary according to the status of the line: electrified or not, 750 V DC 3rd rail (ex Network SouthEast and various metros); 1500 V DC usually overhead (France, Netherlands); 3kV DC (Belgium...);15 kV 16,66 Hz (Germany, Austria and Switzerland) and 25 kV 50 Hz (England, France...).

Usually, over non electrified lines, or DC electrified lines, 50Hz track circuit was used.

With AC electrification, systems have evolved to higher frequencies, of course off the harmonics of the traction current. More recent systems use periodic impulsions of high frequency (something like three impulsions at ~150Hz every second), process which proved to be efficient short sections of rail in dirty environment (leaves, oil...) especially in station areas.

Coming back to your solution, you mentionned a number of external sources of noise. DC usually does not create noise, provided that it is properly filtered in the substation, but DC trains are now equiped with thyristors which "cut" periodically the current in order to deliver the adequate power to the motors. The frequency they cut is around 400 Hz and is perfectly audible on recent trains (jubilee line for instance). Your system will have to avoid these frequencies.

Last question: if you process the signal on the reception side, how long will the process take, and how long is the free distance that it will represent ?

Good luck! (In case your project is not yet done, which I -now- doubt, according to the date of the former posts!!)