App keeps extrapolating indefinitely

[pbock]

When the app can't get a fix because the onboard server doesn't respond, the app just keeps extrapolating the train's last known segment. This is a good idea to a certain extent (in case the server goes down for a bit), but after a minute or so it just becomes too much guesswork. It also means that the app will happily calculate a position as long as the browser window stays open, even days after the user has left the train.