"The electric motors generate a very big magnetic
field that can be detected by a coil..."

That won't work for Harjit, cuz in India still Steam Locomotives are on the track.
<small><small><small><small>as a matter of fact in Pakistan too,  on our side</small></small></small></small>
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So let's decide on something non-magnetic only.

I think nothing is like more effective than the strain guages as already suggested.

Babar