Piling up rocks on a railroad track.
What could go wrong?
Better make damn sure there isn't a train coming.
Engineers get pissed when kids put pennies on the tracks to flatten 'em out. I doubt they'd be too happy with piles of rocks built up next to rails for some guy to ride his bike over.
Pretty sure the NTSB wouldn't look kindly upon it either.
Then again, nothing is illegal unless you get caught