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One of the reasons drivers tend to earn so much (in my experience, at my company) is because there are not enough drivers so they are all offered fairly regular "overtime" at double rate to meet demand.
So even the places that offer good service don't have enough drivers.
It's not uncommon for there to be action short of strike and for the drivers en mass to reject any overtime, this brings services to its knees.
I'm fairly certain thee's a report out there somewhere detailing the lack of driver training funded by the ToCs. This meant that accredited drivers , where they chose, drifted towards the 'better' train driving jobs, and poorly managed franchises like Southern soon found they could not cover obvious items like roster changes and holidays.
There was no pool of qualified labour available.
During the recent RMT strikes the agency companies publicly stated that they were not in favour of supplying labour, not only due to the strike being legitimate, but also the agencies still had no pool of labour to offer.