On the train?

In the script I studied the other day, there was the following sentence.

"I can't believe how much Japanese people read! On the train, in convenience stores, in coffee shops, everywhere."

"On the train"
"In convenience stores"
"In coffee shops"

Only "train" is different from others. There is "the" and singular. Why don't you say "in trains", like others?

Very strange thing for me is "on" the train. My image from this word is we are on the roof of the train. We are inside the train, aren't we? Why isn't it "in the train", but "on the train"? I can somehow understand "on the plane" though.

My iamage of "on the train"