Coins on the Rail
I discovered in my well-spent (?) youth that you have to sellotape the penny onto the track, or the vibration of the approaching train rattles it off the track before the train arrives. I was informed by a classmate that this depends on the speed of the train, and that faster trains get to the penny before it has time to rattle off. My line (Huddersfield to Manchester up the Colne Valley) didn't have any particularly fast trains, so I couldn't test this theory, but I suspect it was bullshit anyway. I strongly suspect said classmate had never put a penny on a track in his life, much less on any line that had fast trains.
I have a confession to make. I couldn't afford a half-crown, but I was the fool who was brave/stupid enough to place pennies (with sellotape) on the rails, so a wealthier classmate provided a half-crown. I invested a two bob bit. Aforesaid classmate was none the wiser: short of weighing it, a squashed two-bob is indistinguishable from a squashed half-crown. Profit: 6d.
When I reported this many years later, another friend said, “My old man was always adamant that they couldn't find them afterwards and that they ended up stuck to the train wheels.”
I suspect that they ended up lost down amongst the ballast, having rattled off the track.
My flattened ones – after I realized about the need for sellotape – fell off the track after being flattened, by which time they were big enough not to disappear down the gaps between ballast stones.
The sellotape did disappear. Whether it was stuck to the wheels or merely turned to mush, I don't know.
In those days, pennies and halfpennies were made of a copper alloy. Nowadays pennies and tuppences are copper-plated steel. I expect the new ones flatten just as well as the old copper ones – rails and rolling stock wheels have to be a much harder steel than coins need to be – but I've not done the experiment...
Snopes has (or had; I've not checked recently) this to say: “Pennying the tracks can result in an additional danger: sometimes those coins shoot out from under the train’s wheels at incredible speeds, turning them into potentially deadly little projectiles.” I think Snopes has been taken for a ride on this occasion: I don’t believe it for a minute.