Recently, I was thinking about railroads. I like railroads; I like train stations. Train stations are a model of simplicity: there is a room for waiting, and an infinite line that passes through. This is very unlike bus stations, which are spiky points where buses must pull in, then back out. Train stations are all about continuous flow.

Train tracks too are models of simplicity and power: two parallel metal bands held fixed apart by crude planks of wood.

The pieces of track are fitted together by a piece of metal that is bolted on with square nuts, almost as big as the length of my thumb. The tracks are nailed down into the ties with spikes whose heads are just as big.

Here's a puzzler for you: do train wheels fit down around the inside or the outside of the track? And why?