On 2012\03\15 21:03, Denis McMahon wrote:

I wouldn't trust a gps derived position underground even if I could
receive the signals - you don't know how much bouncing about it's done
getting through the soil, pipes, rocks of various types, cables etc above
you, and every signal bounce is a loss of accuracy.

GPS is not accurate enough for laying a surface railway, never mind an
underground one.