Trains are complex machines. Most of the time, they work flawlessly, but things can go wrong fast. Broken knuckles between Cars. Car Derailments, Cars, (Ignoring the warning signals, Crossing the tracks, getting hit) Tornado, ETC. Youtube them if you don't believe me.
A human in the cockpit is going to assess those situations much faster than an automated system can in many of these situations.
You want to stop trains from speeding? screw the cameras, Put GPS in the engine (if they don't have them already), map the speeds of the rails to the GPS and don't let the train go over the speed limit the GPS (or Dead Reckoning if it's in a tunnel) says you should be going. Its been in Semis for years and it would take next to nothing to modify that system for Train engines with no change to the rail infrastructure.