There are several issues with the "rail vehicle" is that it can only go where the rails go. So, say a car breaks down on the tracks, then all vehicles behind it stop. A bicycle is going slow along the route, then all vehicles go slow.
It works reasonably well for a tram system with a fixed route as long as the trams are given priority on the rails, but people don't live by fixed routes.
One doesn't have to do lane tracking, but the autonomous vehicle still must be aware of surroundings, so it stops for the kid that darts in front of it. Road awareness is half the battle.
Perhaps we'll eventually see AV convoys. Clip them together on highways like a train to benefit from drafting and reduced wind resistance. Also, higher density travel.
Autonomous trains? We may well see those in the near future, especially in rural areas. Although, perhaps one shouldn't worry about labor costs for driving 50,000 tons of cargo across the country. However, an automated speed/braking system might have prevented the recent Amtrak crash in Washington.