I must be having a dense moment. I thought we push the fat guy because he has committed the deadly sin of gluttony, and thus some of us would rationalize pushing him would be okay then. But you (or the question) seem to be saying that the fat guy actually has enough mass to have a significant affect on the train's momentum. I was confused because I find the latter hard to believe. Or is there a third possibility: He's so fat that the conductor sees him in time? Regardless, if I'm sure of the outcome by pushing him, he goes. I murder 2, save 5; I get a +3 in the bottom right corner of my spreadsheet for the win!

If, of course, we're talkin' about who these people are, everything's different: Hot chicks and loved ones first, then the spreadsheet thing. But you said the singular person was a dude, and I probably don't know him, so...