This is exactly the kind of Feynman explanation I love. Not only does he make the complex simple, but he seems to be taking perverse pleasure in revealing something simple that few people think about but is an interesting concept once you start thinking about it.
I’m one of the few oddballs who thinks about this stuff and I knew this fact about trains but could never explain it like Feynman.
Twenty five years ago when I lived in Oregon I noticed that the chip trucks: large tractor-trailers with oversize trailers that hauled wood chips to paper mills had atypical wheels and axils on their trailers. The front of the two rear axils with its double wheels was not set just in front of the rear set but in fact, about 1/3 of the way up trailer toward the front. I wondered how this setup went around a corner? Which axil did the turning and did the other one just scrape sideways along the road? In fact, unlike a train with its flanged wheels trucks have differentials but the differentials don’t account for this problem. I’ve asked lots of people about this over the years and the consensus is that the rear wheels “fudge” a bit which means they do scrape sideways some as the truck turns.