Trains are faster if they're grade separated because they don't have to wait for traffic. Not waiting for traffic also makes them more reliable, especially during rush hour. If you can't rely on the schedule, you have to put padding into your schedule, which makes them effectively even slower. Trains are also faster because they have lots of doors and the passengers are waiting at the same level as the doors, and you're not waiting for people to buy tickets, so the stops are quicker. Since they move faster, you can also run a more frequent service with the same number of vehicles (driver cost is a very large component of operating costs of a public transit system) They're also smoother and so more pleasant to ride. You're usually also waiting for them out of the elements.