Interesting. So they did their due diligence. The surprise is that PG&E would be willing to accept the power, especially in a net metering arrangement. They (and most other California utilities, including SMUD) have eliminated net metering 1.0 (credit for surplus power at retail or other final consumption rates) as a workable concept. "Net Metering 3.0" is the current policy, which isn't net metering at all. If the utility accepts a reverse feed at all it'll be at best at avoided cost (fixed-rate subject to periodic adjustment, not time of use), and for commercial entities like Caltrain most likely at variable (sometimes zero) "distributed generation" wholesale. Utility preference these days is that somebody producing surplus power (surplus in real time to their own needs) buy their own storage (e.g. big batteries), manage their own "net metering," and not bother the utility with the surplus.

Caltrain's power purchases, as with most transit lines, will be expensive because of demand charges added to the rates. The trains have to run, even during peak power use times, so those charges are unavoidable (it's not just TOU rates, either - look up a commercial power rate sometime). Being able to reduce demand charges through reusing regenerated power for other trains on the system will be worthwhile whether or not Caltrain can get a reasonable deal on back-feeding surplus.