Perhaps you should approach Bombardier to find out what sort of electrical products are passed into the running rails by a modern train. IIRC they make the 375/376/377 series EMUs that are taking over the South East at the moment.
Link to them at -
http://www.railway-technology.com/co...ier_transport/
Also since what you are doing is mildly similar to radar with aircraft, it might pay you to have a look at what sort of software/DSP is already developed in that field for distinguishing between the 'return' and the 'clutter'.