Except TL can't be run as it was because they have split some of the old services with the 2 halves now being joined to different halves. E.g the Former Bedford- Three Bridges via Redhill has got extended to Horsham and the Southern end but is not joined to a Peterborough service instead of a Bedford service north of the core.
The new timetable will also be required to help provide driver training opportunities.
Bedford to Three Bridges is actually not such a problem, and you wouldn’t need to run it in two halves. In fact, doing so would be a logistical issue. This route has very little to do with Peterborough to Horsham, and was one which has already existed on weekday / Saturday daytimes. It merely ended up modified to be curtailed one stop shorter at Gatwick. The driver and unit resources were basically already in place before mid-May, and are suited to the TL operation.
However, Peterborough and Horsham have been joined up across the Thames by linking semi-fast Peterborough services with the former London Bridge to Horsham route, the latter having now theoretically been completely swallowed up by TL, although other GTR routes are now having to temporarily stop specially at the myriad times with numerous cancellations.
Because of the structure of depots and their driver knowledge, TL are basically struggling between transferring from rather more localised knowledge (Peterborough to London OR London to Horsham) to depots which operate the whole route from one end or the other. Eg. Horsham depot has been split in half, and half the depot will only work north from Horsham, all the way to Peterborough on TL. This needs route learning.
But whilst there are enough drivers to shadow for route learning north of Finsbury Park or south of London Bridge and so drivers are relatively OK learning them (there are always some rolling training programmes on any established route), drivers from both ends are unfamiliar with the Core, so pilot drivers in the Core are more stretched, as everyone needs showing the route.
Ditto Rainham to Luton and suchlike.
That's not how route knowledge works - or are you suggesting that this farce will go on long enough that someone who gets New Cross Gate to Blackfriars (perhaps the most understandable route) won't do it again for so long they lose it?
Quite possible, actually. Acquiring and then losing routes due to not working them is definitely “a thing”.
I concede it’s unlikely on Thameslink, but you never know...