Depending on area some might move to other TOC's for either more pay or different work .
There are progression opportunities available through the management structures , be it Driver Manager / Lead Driver Manager / Operations Manager / Head of x / etc .
There are also safety/standards compliance roles that someone with the right kind of experience and skill could move into from train driving
Some might also see moving into operations training as a driver trainer as a form of progression .
That being said a lot of those roles whilst the basic wage might be slightly higher than driving , the earning potential from overtime enhancements afforded to drivers means in reality you can make more as a driver without having to take the job home with you . Which might be more an expectation in those other roles .
But its not your typical corporate career ladder job either so as already said the majority stay as drivers .