Interchange detection needed for forks

[twpol]

Some lines have forks in, e.g. Metropolitan, which means that routing from one fork to another needs to head down the line, change direction and head back up the same line, on a different route. In these cases, the best interchange is obvious: the first station where both relevant forks are joined.

Currently these stations are not detected as such, leading to silliness like this Amersham to Chesham route taking 80 minutes at best - where in fact, it should be closer to 14 minutes.