Andy Neil wrote:
-------------------------------
"how about a gap in the trach shorted by the wheel?"



and we were worrying about cost - I hate to think how much it would cost to get a gang out to cut this gap into the track!!

(I'm assuming we're talking real railways here!)



And wouldn't this mess up the signalling, and possibly also the power return on electrified tracks?


Actually, this is the standard way to detect train location. It's called "shunt". Tracks are divided into blocks, and each block is "protected" by a signal. When a train enters a block (passes a signal) it rolls over gaps in the track, and the axle and wheel assemblies shunt the rails together in the next block.

There have been some documented cases of loss of shunt, but the shunt method is considered to be acceptable for block protection (safety-critical).

Other detection methods including GPS are now being applied, especially for communications based train control.

Getting approval for a different method will be almost impossible. Signal engineers and railway operators will only accept technologies that have been thoroughly field proven to be reliable for safety critical applications.