Thanks for posting that image.
Now I know what I was probably looking at several years ago in the middle of Nevada. Along the Tonopah Ry at (I beleive) Goldfield Junction there is a similar concrete pit. The size is such that it must have been from the narrow gauge days and not the standard gauge Tonopah & Goldfield. The person who told us about it thought it was a engine pit or drop pit -- but it was too close to the next track alignment for that purpose; and just didn't look right for that purpose.
By the way, the Central Nevada Museum in Tonopah has the scale house from the track scale (standard gauge) that the SP had at Mina. I cannot recall if it has any of the scale mechanism with it.
Brian Norden