Getting it while on a train. The railroad version of the Mile High Club. Taken from the term "mile post," the railroad equivalent of highway mile markers. Applies to intercity and commuter trains (Amtrak, Metra, Metro North, etc.).
1. I joined the milepost club on the Amtrak train to New York last night.
2. Sex life dull and boring? Try riding the train and joining the mile post club.
3. You've banged 3 girls on a plane? Well, I banged 7 girls on Amtrak on one train trip from New York to LA. The benefits of being a member of the mile post club.
4. I never had time to pursue my sexual fantasies before I joined the mile post club. Now I get it every night on my commute on Metra from Chicago to Homewood.