I don't know what concept of time you're using, but if you're traveling from one city to another, I'm 99% sure a train will be faster simply because it doesn't have to deal with traffic. The time it takes isn't as important either. One train can hold many more people than a bus and that's much more important.

Buses cost less at first, but then you need many more of them to accomodate more passengers. Their speed is also limited and varies quite a lot.

Haven't you ever played that game Transport Tycoon Deluxe? You buy one train and it makes waaaaaay more money than 4-10 passenger buses!