If I put a penny on an RR track and a train runs over it, it’ll be a thin piece of metal when it’s over. I’ve seen kids do this. I may have done this myself as a youngster, but being retired, I don’t remember that far back.
So, how much energy does it take to squish that penny? If I took 10 rolls of pennies (500 total) and laid them on the both tracks every foot for 249 feet, would the train’s speed diminish noticeably?
Every once in awhile I still see those penny smashers (excuse me, “coin operated manual crank penny press machines”) that stamp the logo of that particular tourist trap, so the energy needed is small enough that it can be supplied with a few pulleys and gears an 8-year old can handle.
These guys claim their penny smashers deliver 22 tons of pressure. I found one cite for a locomotive engine that says it weighs 134 tons.
About 10 years ago trying to be the cool uncle when I tried this, I put the 6 or 7 coins I had in my pocket a few feet apart down the length of a rail. After the train passed we searched and searched, and finally found one flattened coin several feet away.
They probably don’t want people running out to the tracks to place coins just before the train rolls past. Hitting people causes a lot of paperwork (glib answer) and is very traumatic for all involved (serious answer).
The coins aren’t going to have any effect on anything rolling on the rails, from a scooter to a heavy freight train.
A standard part of the tourist ride on the Eureka Springs and North Arkansas Railway is flattening coins on the rails - passengers de-board at the wye at the end of the line, and while the train is turning around, the conductor and guides tell people how to position coins for flattening. They say the secret is a goodly amount of spit. The coins are still quite hot after being flattened. Showing people this way is good - everybody gets their coins placed and then the train starts moving, so everybody is safe.
Using the work-energy theorem, with some back-of-the-envelope estimates: An intact penny is just about 1 mm thick, and a flattened one is maybe a quarter of that. So we’re applying a force over a distance of 7.5e-4 m. Using the figure of 22 tons for a penny-smasher (which is going to be comparable to that from a train car, since not all of the weight is on one wheel), we get about 220000 newtons for the force. That gives us a total energy of 2207.5e-4 Nm, or 165 J. For comparison, this means that if the handle of the smasher moves about 8 m (that’d be four full rotations, with a handle about a third of a meter long), then the person turning the crank must exert a force of about 21 N (about 4.6 pounds), which isn’t too unreasonable.