I hope you do realize that you're saying a tiny, 2mm thick piece of soft copper alloy can derail a virtually unstoppable behemoth that weighs at least 3100 tons (we're talking about your average freight train, cca.2004)... It's impossible, the copper alloy can't withstand even the weight of a 4-axle 140 ton locomotive, let alone a whole freight train - not like it matters, the penny will be turned to a thin foil after your average 6-axle freight loco goes over it. I've tried the penny myth years ago at a local railyard, with a large array of coins, metal pieces and rocks. The only way you could derail a train is to take a solid metal wedge and glue/weld it on the track. Once (if) the axles leave the track, they're most likely going to slide to one side (they're not fixed by gears, they're on a ball-bearing socket) and then ka-boom, say hello to disaster. Don't try this though, it would not be recommended.
So no, you can't derail a train with a penny. But you can sure make some pretty shiny discs out of it :3
