So he's using the muni gem[1], which uses the nextmuni/nextbus api. To my knowledge, NextBus is unaware of the length of trains. The announcements in subway stations here are sourced from the Automatic Train Control (ATC) system present in the subway. If you're in a subway station, it's the screen sometimes next to the nextmuni screen -- the one with the terrible mspaint graphical style showing the length of the ATC system.

[1] https://github.com/jeffremer/muni