I'm not an expert by any means, but your description of the longhorned borer sounds more like what I've seen around here. The description of the adult is a better fit and the holes in the wood are round.
I will state that this picture is of the largest larvae I've seen; most of what I see in the wood are similar in form but only about 1/8" diameter and perhaps 3/4" long.
Lots of bug-killed trees here north of Boise.
The connection to railroads (per Ken in Buena Vista) is that the Intermountain RR came through here as a logging RR and likely used this type of wood for ties