There's a world of difference between a DLR type metro operation and a main line railway. On a DLR type operation, all the trains are exactly the same in performance, all stop at every station and do not run a high speed. On a main line railway there is a variety of different traction of different performance characteristics, stopping patterns vary and there are big speed differentials. Even on newly built high speed lines where all the trains broadly have the same performance there is still a driver on board to actually control the train. In part this is because few high speed lines are self contained, they start and finish their journeys on conventional lines (the main exception to this is Japan where the high speed network is standard gauge and the main rail network is narrow gauge).