Train can carry a very large number of passengers. To carry all the passengers, we will need a large number of buses which will consume more fuel. So train is better.
Also train can run in electricity and we can save coal by this. Train can also transport large amount of goods using less fuel compared to when a number of buses are used to transport those goods.
Train as a transport conserve more fuel, when compared to that of the bus, it is because, bus just needs some liters of diesel for moving, and it carries only small amount of the passengers. But, train have to pull many compartments, and the train with diesel engine, i.e., running in diesel, needs more amount of diesel than that of the bus, so the fuel conserved by the train is more when compared to that of the bus.

Thank you.
Hope this will help you.