In message , at 10:25:02 on Fri, 16 Mar
2012, Roland Perry remarked:
GPS is not accurate enough for laying a surface railway, never mind an
underground one.

It's accurate to about 10cm (if you employ differential GPS) which
probably good enough for avoiding a 10m obstacle 40m underground.

With an implied "apart from the fact it doesn't work under ground", of
course.

Obviously, you don't use it to measure the distance between the rails
when you are laying the track.

--
Roland Perry