Most large cities these days have some sort of metropolitan commuter train system. What are the reasons for the commuter trains, why couldn't we instead of having train tracks why not clear a large distance of straight road and have a vehicle, such as a large bus riding on it?
Looking at the speed of trains (in my city they go at an average of around 70 km/h) surely the speed alone is not a factor since it would probably be feasible to have some sort of large vehicle achieve this speed. Trains also require large infrastructure such as tracks and power lines. On the other hand, such large vehicles would probably be safer on tracks instead of roads, and a train on tracks would probably be easier to automatize than a vehicle with wheels.
I can think of a lot of reasons for and against trains over wheeled vehicles, but what are the real reasons why commuter trains are so popular a choice for cities?