IPT doesn't support that.

That's a shame. Thanks for the reply :D

Advanced Vehicle Options allows this.

You can even disable certain trains but still allow those trains on your internal routes.

i played around with AVO, but I don't see an obvious option where I can customise for external lines

mono, "allow vehicle to spawn" sets which vehicles can and can't spawn in the wild. That way if for instance you want a certain local train to only run on your suburban train system but not see it spawning elsewhere you could set it as such.