1. There are plenty existing train systems where large trains are formed by coupling together multiple small trains that all have their own engines. That way, train lengths can be varied to optimize for demand. This wouldn't be happening if it were prohibitively expensive.

2. Why would you need to avoid crossings with roads just because it's autonomous? Trains use signals that only turn green when the section ahead is clear of other trains or vehicles. It's not like train conductors check road crossings by sight to decide whether to drive or brake.