Look at the populations. If UTA actually does serve a population of 300K (I think SLC's much bigger, but don't know how big), it's comparable to SMART. Santa Rosa alone is near if not over 200K. Up to date loco-hauled trains should work fine, and by definition (using standard designs) meet FRA standards. Though it might be a good idea (if it's not standard by then) to get the crash energy absorber car design that LA Metrolink has on their new cars (... beat 'em with enough crash experience and eventually learning happens?).

The reason rail can't get to Marin City is that the line south of Corte Madera has been turned into a very popular bike/hike trail. And of course parts of it have been built on and the tunnel to Mill Valley if not collapsed would require jacking up the idea and sliding in a new tunnel to work with modern rail of any kind and you'd need a new bridge at C.M.Creek and ... Fugedaboutit.

When I lived in the North Bay, the commute express bus (74?) from Santa Rosa to SF was scheduled at about 25 mph average. That was realistic on normal traffic days; on bad days especially Friday afternoons all bets were off (worst day was nearly 4 hours SF to SR, which approaches the total time of a NWP ferry and local train of the 1930s). That may be better now that more of the HOV lanes are done. Commute rail has to beat that to be competitive; otherwise, the only selling point is relative reliability of schedule, which doesn't go too far.