Don't forget the famous exchange trials which saw all of the big four's express locomotives competing on rival turf officially.
Plus the various swaps between companies for various reasons,
Pendennis Castle operating out of King's Cross for a while in exchange for
Victor Wild (I think) going to Old Oak. The LMS also had a
Castle for a while. Apparently, there was a bit of industrial espionage during the
Pendennis visit, resulting in exact drawings of the Castle's long-travel valve gear appearing on Gresley's desk at King's Cross, hence the valve gear modifications to the A1s.
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Didn't the LMS even try to get the GWR to build them a batch of Castle class locos? - A request which was turned down!
Owing to lack of capacity at Swindon, where they were trying to build the
King class at the time. In the end, the Southern provided some drawings for the
Lord Nelsons, leading to a decidedly Nelsonian look to the
Royal Scots, despite their obvious differences (three cylinders instead of four and so on). I've often wondered if the number 6100 was deliberately chosen to be a little higher than 6000.