It seems there are several issues here. Given that this is commercial rail
and human lives are at stake (at least here in the USA) the dominant factor
should/would be safety and liability. The cost of the inevitable lawsuit is
much, MUCH greater than the cost of this system. Given that there are
available, existing, accepted mechanisms for detecting trains (e.g.
crossing light controls), I'd be very reluctant to touch this. Assuming
that either India is different or that this is an intellectual exercise....

There's seems to be two options here, remote sensor 2km away or a _really_
sensitive detector that can recognize a train 2km away. Granted a remote
sensor has issues - power and communication mainly. Those seem readily
surmountable with off the shelf product. Solar and batteries, RF link, lay
a cable, induce the signal into the track.

I think the killer problem for the really sensitive detector concept is
people. Several sensors have been suggested that _could_ sense a train from
a distance in a controlled environment. But inverse square law and common
sense says that any detector that can sense a train at 2km is going to be
swamped by a bicycle at 1m. Multiple sensors and complex filtering _may_ be
able to discern signal from the noise (rhythm of the train going over the
track joints?) but complexity is rising as reliability is falling...

-Denny
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