I would guess that a train is much harder to stop than a truck. Further, one can drive next to a truck and have people jump on the truck to do some "snooping." The "driver" is not going to freak out/swerve as the drone truck just keeps going towards its destination at a perfectly efficient speed, perfectly centered in the lane.

In plenty of places, you can stop a train simply by cutting the signal wire (because it will fail-safe).