Re: Why one narrow guage versus another?

What I have long wondered is why coal mines in the eastern US (and perhaps in the western US as well, I don't know) tended to use 42" gauge.

There were certainly exceptions (for example, the EBT used 3' gauge in their mines, and the Hudson Coal Co. was 30" gauge), but 42" gauge was nearly universal in the Appalachian coalfields, and it appears to be have been so from a very early date. (I recall that Baldwin's first narrow gauge locomotive was a 42" gauge engine built for a West Virginia coal company, all the way back in 1868.) Was this just a matter of folk custom or convention, or was there some valid engineering reason behind it?

-Philip Marshall