A-HA! I think I have an answer, as I remember recently reading about this very topic!
As explained by Carl Swanson in the August 2007 "Model Railroader," p. 20:
"A pike is an ancient infantry weapon, a pointed metal head on a long thin wooden shaft. In the old days, private companies built toll roads, which were blocked by a wood shaft - a pike. You would pay your toll, and the attendant would swing the pike away to let you through. Toll roads are still called turnpikes today.
"In the past, railroaders often used 'pike,' in the sense of 'road,' as slang for their full-size railroad. Modelers picked up on that and started calling their layouts 'pikes.' ..."
I don't know any better, and that sounds as plausible an explanation as any to me. I'll buy it for a dollar!