If self driving succeeds, it is because it is safer than a human driver. If nothing else, it'll cut down on insurance premiums, and increase fuel efficiency by reducing speeding and unnecessary hard acceleration.

There are also a lot more people currently driving than there are existing train drivers (which you can see I was comparing existing infrastructure) so the time tradeoff could easily lead to increased productivity, however you choose to measure it.

All of this, of course, assumes fully autonomous driving. I'm somewhat sceptical that we'll actually get there, myself.