One train can have a capacity of 1000 people. If you run these at 3 minutes interval, you have a throughput of 20,000 people per hour.

One bus takes less than 100 people, and you cannot run 200 buses per hour. Actually about 20 buses per hour is pretty much at the upper limits.

These are rough numbers, but if you need to move more than 2000 people per hour, trains can have the capacity but buses don't.