I asked a guy at work if he had ever put coins on a railroad track when he was a kid. He replied jokingly that the train would derail. So then I said, what if you had a linear accelerator or something and launched a penny at a large fraction of lightspeed, hitting the train from the side? How fast would the penny have to travel to gain mass sufficient to derail the train? Or would it just punch through one side and out the other?
Neither. At some velocity far below c, the penny and a small area where it impacted would vaporize as the impact energy is dissipated as heat.
It would punch through - the way to imagine it is to ask whether there’s any part of a train that’s no bigger than a penny, but would be strong enough to withstand the instantaneous transfer of sufficient energy to derail the train. I don’t think there is any physical material strong enough for the job.
A 2.5 gram penny moving at 10% c has a momentum of 75,000 kg m/s
Diesel locomotives range in mass from the lightweight 22,680 kg GE to the 113,400 kg GE.
What happens to a locomotive when another locomotive slams into it at 1 meter per second? I expect nothing good.
Strictly speaking, at that speed the penny is going to disintegrate in a bright green flash due to heating from ram pressure and parasitic drag in the atmosphere long before it hits the train.
Stranger
We can pretend there’s no atmosphere, but what’s “ram pressure”?
Peace
mangeorge
Basically, when something moving fast scrunches up a bunch of air in front of it, creating a high pressure in front. Personally, I’d lump it in under “air resistance” or “drag”, but some folks prefer to reserve that term for viscous effects.
Having grown up a few blocks from some busy railroad tracks, I can honestly say that it is virtually impossible to derail a train by placing anything on the tracks. Coins of all denominations, metal washers of various sizes, a variety of different bullets all the way up to a .44 magnum, 1/4 inch steel cable, and a selection of toy cars and trucks. Most were flattened, a few flew forward and some were basically pulverized. A few things, such as larger nuts and bolts, would cause the wheels to slide on the tracks but only for a short distance.
FYI, none of the bullets placed on the tracks fired with enough force to actually shoot the slug. When placed with the slug facing forward, most of the time the case would be flattened and the slug would harmlessly pop out. Occasionally the gunpowder would ignite with a puff of smoke. When placed with the slug facing the train, the slug would be flattened and the gunpowder would pop, likely from the compression of the shell being smashed flat.
FYI #2, we tried a variety of methods to keep the items to be smashed on the tracks. We found rubber cement worked the best, things would flatten without a foreign substance getting smashed into the item. We also used tape, bailing wire and other types of glues.
I see. So “ram pressure” also requires atmosphere. In a vacuum, the coin would remain intact until impact.
Hmmm. Wouldn’t the molecules closest to the starting point be moving faster than those farther away during acceleration? That would cause some heat from “squishing”, wouldn’t it?
Kinda like an object approaching a black hole, but opposite.
Thanks
A nail makes a pretty cool facsimile of a sword. 
racer72
When I was a kid, if I had known you, you’re the guy that I would have been hanging out with.
That is, a nail flattened on the track by a train. We’d lay a fairly large nail, the size used for framing or larger, lengthwise on the track. After the train flattened it we’d wrap string around near the head and glue it to make a haft. It would hold a fair edge, good enough for throwing.