I don't think I kept it, but a couple of years ago I made a video of a £1 coin (as thick as two US pennies) balanced in a shallow groove in the table on a long

I don't think I kept it, but a couple of years ago I made a video of a £1 coin (as thick as two US pennies) balanced in a shallow groove in the table on a long distance train in Denmark. It was about 30km before it fell over. (But maintaining the track to this quality would be even more expensive if there was as much freight on it as tracks in the USA carry.)