How to measure rail-to-earth resistance and rail-to-earth potentials

[tatigo]

Dear forum members,

Would you give please a practical tips how to measure and what measurement equipment to use (with some examples if possible) to measure the rail-to-earth (tunnel) resistance and rail-to-earth (tunnel) potentials?
The measurements needed to be done in the tunnel (cast iron tubes) and above ground also (soil and ballast under the tracks) ?
It is DC traction metro.

Thank you so much.

 

[ActionDave]

I don't see how a forum that deals primarily with building wiring in the US can be of help with this.