a PBS? bug with 2 trains (hard to explain here)

[SHADOW-XIII]

Well I do not have a save now so maybe someone could reproduce it

the problem is that:
I got a junction and after it a hill

there are 2 trains, first train is heavy (freight=F), second is fast one (let it be P)

F train goes very slow going uphill (around 1kph)
F driving slow is leaving the PBS junction but
P train somehow kick F train back ... and cabooom

I was thinking how it could be ... the only thing I came up is that the track unshade (is free at PBS track) too soon (before train really leavs it) so 2nd train can catch it (if the 1st train is really slow and 2nd one fast enough)