there are driverless trains at lots of airports, but i think the reason they dont appear elsewhere is that the cost savings wouldnt be as great.  a single train conductor is managing a massive vehicle - sometimes two orders of magnitude more than a truck.  the labor component with trains is significantly lower than in trucking.  i think the same is true for ships.  taxis are at the other end of the spectrum, where labor is a very large part of the cost structure.