This is another one of those don't be an idiot and watch out for idiots chapters. Children and people who aren't thinking straight, sometimes come up with the brilliant idea of placing a coin on a train track.
This will result in the coin being completely flattened (might need to be taped down depending on coin type or it'll be shot out). Other objects such as small pebbles could also be shot out at very dangerous speeds.
Basically, if the object is too big, you're going to derail the f-,%*ing train. If it's a smaller one, it might be shot out and seriously injure someone.
Of course the likelihood of being hit by one tiny projectile isn't that high, but what if there were multiple.
Also, based on the law of Murphy, you'll will 100% be hit a coin in the eye, instantly destroying it and causing lasting brain injuries. This will result in half your body being permanently paralyzed.