Reference the earlier comment that it would have been a lot cheaper decades ago might make sense from a public policy perspective but SP was so eager to discontinue the commute service that they offered to purchase vans if they could exit the business.
A lot of money would be needed to keep the system operating long term (new locomotives, perhaps cab cars, station repairs) and this seemed a better approach. At this time the trains were being handled by Trainmasters, GP9's the SD9s so loved on this forum and the lonely FP7 #6462. (The SDP45's eventually were made available and SP did end up purchasing three GP40-2'P's.)
All trains had to be switched at San Jose and San Francisco to position a locomotive in the lead and there was very little "reverse" commute traffic. Replacing the fleet and adding electrification was clearly out of reach.
TO Discussion of Vans