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1.
when 3 or more guys all have anal sex together in a row
i had a bum train last night, there were 8 of us
by robin November 11, 2003
 
2.
Line up and take it up the bum! 2,3,4 or better still more, all get involved in the Train of Bum. It's not Gay, it's just constructive exercise between mates.

Anyone can make a woman scream. It's takes a real man to make a bloke scream like a little boy who's been stung on the finger by a wasp!
Instigator: "Who's up for a Bum Train later on tonight?
Everyone else: "Too right, rate 'ard."
Instigator: "Bags' in middle!!!"
by Marv Sharpe March 19, 2008