Route Learning has to be done on in-service trains on busy infrastructure - which is what the Thameslink core is, route learning saloons aren't an option. Re your point about every secondman's seat being filled, that isn't necessarily the best way to do it - GTR are still trying to run trains in between the route learning, and filling all of the second seats means fewer drivers actually driving trains. Similarly, not every train is going over useful track for learning - the issues AIUI are mainly GN drivers through the core and down to Rainham - Wimbledon loop services (as an example) are of use to neither.
I would direct you to the
London Reconnections article on this debacle, which is in depth and informative. I will pull out a couple of highlights that are relevant to your first point:
"This lack of drivers actually predates the GTR franchise. Both the previous incumbents (First Capital Connect and Govia’s Southern) informed the DfT almost six years ago that the department would need to fund a ramp up in driver recruitment and training to prevent a major driver shortfall. They were informed then that would prevent the services being run as intended both during, and after, the London Bridge rebuild."
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"Although some of the blame for the above will likely be laid at GTR’s door, to a certain extent they are also victims of issues at the DfT. The biggest indicator of that is perhaps that many of the driver issues can be traced back to the GTR franchise bid having too few drivers specified. Yet this is partially the result of a DfT error in the numbers given to bidders. GTR’s assumptions were based on those numbers."
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