I was in Tokyo, Japan with some friends of mine (only in the dream, I didn't know them in real life) and we were challenging each other to try and make it from one subway station to another without boarding a train. This meant travelling along the tracks.
The tunnels were wide with at least 6-7 lines running through them with plenty of places for trains to cross to a new line. The trains came frequently with some lines seeing a new train every 5 seconds. It wasn't too hard to travel down the tunnel, but you'd sometimes have to cross a line due to a merge point ahead, meaning you'd have to time the crossing well. For some reason there was no stopping once you started down.
I ran down the tunnel while all kinds of trains shot by, from single car commuter trains to shinkansen. Reaching the next station, I was spotted trying to merge with passengers getting off a train and had to pay a $30 fine. This also happened to all of my friends. Passengers were incentivised to spot and call out anyone running alongside the tracks in this manner with devices for this purpose installed on some trains (they looked a bit like telescopes).
We couldn't be bothered trying to cross the language barrier when trying to pay at one of the station's service desks, so a friend loaded a previous save and we tried the run again with a different route which resulted in us not being caught this time.