Named after the New York City Subway lines, the P train is a mythical train whose riders demonstrate behavior which labels them a pussy. Most frequently used to describe a person who leaves the bar first or without sufficient reason when out with friends or colleagues. Also applicable to someone who exclusively drinks lite beer. Unlike the non fictional Subway lines, whose letter or number designations do not stand for anything, the P in 'P Train' stands for pussy.
Anthony: How many Amstel Lites have you had tonight?
Andy: One and a half, I'm wasted. I need to leave now.
Will: Its only 7:30, Andy's definitely riding the P Train tonight.