My two cents: If by "Tejon" we're talking about a route that parallels I-5, Santa Fe did a study of a similar alignment 100 years ago. They reached the conclusion that it was possible from an engineering standpoint, but it was cost prohibitive. Lots of tunneling would be involved, although not as much as many would expect. The problem back then was that there were not enough people and not enough freight to justify the cost.

Back then, geology was also poorly understood by present day standards. Undoubtedly Santa Fe's engineers knew there was a seismic risk, it's likely they had no knowledge of concepts like strike-slip or blind thrust faults.

Even with today's modern science, a trip underneath the Tehachapi and San Emigdio mountains would make me a little nervous.