Fun fact: you know token ring networks? It's based on a system of reliable mutual exclusion used by early railways.
It worked like this: you have a long stretch of single-track line. Trains run in both directions on the line. Obviously you want to prevent collisions. But you don't have communications from one end to the other. How?
At each end of the line, by the signal which lets trains onto the line, there's a hook. There is exactly one token, which is a physical object which hangs on the hook. A train is only allowed onto the single-track line if the token is physically in the possession of the driver. On emerging from the other end of the line the driver hangs the token back on the hook.
Simple and foolproof (if not terribly efficient).