for trains: stop when you hit the big concrete wall
for everybody else: stop when you still want to do some more on it.
Because after you go too far, you will be left wishing you could do less, and you never can.
bum piss factor: zero, but this is a favorite spot for people to bring their dogs for a run so the dog poo factor is around 8 or 9.
Funny how bums steer clear of places where people walk their dogs.