I had an afterthought on that, and changed Southern Coastway to Arun Valley as a result! I agree that going via Redhill and Balcombe doesn't work.Also consider the track layout - it's paired by use from Balcombe Tunnel Jn, with the Crawley line joining on the slow line.
You are right, but, on the other hand, the Arun Valley trains would no longer be doing this at Gatwick.Even though they emerge at Three Bridges on the "Redhill" side of the formation, sooner or later they have to cross to the Victoria route, whether at Stoats Nest or Selhurst if they go via Redhill.
Those services are often full and standing by Gatwick even outside peak times, they need to run via the Quarry lines.
The solution to overcrowding is longer trains, irrespective of the route they take.I think the suggestion was that the Portsmouth / Bognor services off the Arun Valley go via Redhill, but that is not a good idea, for the overcrowding reasons you set out above.
You are right that the slow line stations between Hitchin and Peterborough are one station bank, and it is very difficult to change that, especially since the 2025 ECML timetable.The thing with Peterborough is this - the slow line stations between Stevenage and Peterborough are one station bank (economically and operationally). If Arlesey is a suitable Thameslink location then those Thameslink services have to run to/from Peterborough.
But Arlesey is the tail, not the dog. What matters is what's suitable for Peterborough, Huntingdon and St Neots. And, if Peterborough isn't a suitable Thameslink location, then that means that the whole station group isn't suitable for Thameslink, even if Arlesey is suitable.
We are agreed on that. Rainham was only put into Thameslink in the quest for the 24tph holy grail.And all MML Slows to Elephant. Rainham needs to go - it was a last minute reactive change as Windmill Bridge needed to spit out 2tph.