(Click to enlarge)
As near as I can ascertain from readers' assertions and web links, this is an actual picture of an actual poster that many claim to have seen in person on the MARC commuter line in at least two separate locations. It's posted at a rail photo site as well. So it sems fairly safe to say that this exists ... which leaves the question remaining of whether it's genuine, i.e. whether it was placed there by the appropriate authorities in earnest (and someone really did think the blatant Stalinist Soviet imagery was a good Homeland Security tactic).
That's what you get to decide!
Real homeland security poster or guerrilla art?
If you have further verification of its authenticity or lack thereof, post it in comments. If you don't, vote before you read them.