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1.
When 2 or more people are riding the train together multiple times for an extended period of time. They bond, and share experiences. They become best friends!

They always need a nickname.

(They become better friends if all of them ride the train EVERYDAY instead of parents driving them.)
Alright sounds good, we can be train buddies! what should our nickname be?
by gonarbs July 19, 2010
 
2.
A stranger that is frequently on the subway, in the same train car, with you. This typically occurs on a morning or afternoon commute.
I know that guy. How do I know the guy? Oh, he's my train buddy.
by Shivonna December 11, 2013