A euphemism used to describe getting the raw end of a deal. This can be used in place of getting screwed, getting fucked. The term originated from the common cover "We're going on a canoe trip!" used by couples to mask the fact that they're just heading to a cabin for sexual relations.
Scott: "Hey Will, how's your canoe trip?"
Will: "Getting railed. They're fucking me, brother."
Wife: "Honey, how was work today?"
Husband: "It was a real canoe trip."