I would echo Gareth, in that Arriva staff are generally very diligent in seeing which portion you need to be according to your ticket after Shrewsbury and advising the walk along the train where necesary, since leaving Mach on the wrong bit could be disastrous.
In extremis if you leave Mach on wrong portion, you can sometime run across between the portions at Dyfi Junction, but this cant be relied on, e.g. if they're running late the aber portion wont wait for the Pwhelli portion at Dyfi Jn.
PS whilst we're on the Cambrian, I've been complaining to NR about the poor state of the track (potholes big enough to swim in) to Dyfi Jn (currently only pedestrian access). NR reponse was something vague about looking into vehicle access, anyone else heard anything? Might be a case of wait and design junction with the A-road looks once widening is finished?