regarding why did use on train and in bys

"on a/the train/bus" -- this is the normal wording, e.g. when someone is using the vehicle to travel in the normal way

"in a/the train/bus" -- could be used when there is some reason to emphasise "inside", or in some other special situation (e.g. "He lives in a bus")

"on/in train/bus" is wrong

When we talk about our method of traveling, we commonly say 'on the train' and 'on the bus'.

Clive

On:
He is getting on the bus.
He is getting on the train.

He is on the train to Mumbai.
He is on the bus to Mumbai.
He is on the ferry boat.
He is on his motorcycle.
Ram is riding on his bicycle.
Ram is on the airplane.
Look at all the people on that bus!

In:
He is in a taxi.
He is in his car.