Yorkshire Dave
Western Thunderer
My understanding (not necessarily correct) is that locos were changed (LNW/LBSC) at Willesden or thereabouts on the Sunny South Express. Indeed, somewhere I have a vague recollection of reading of some comparative tests that took place when the LBSC loco continued northwards over the LNWR (possibly related to superheating). I assume an LNWR loco would be too tall to run on the LBSC as the LNW loading gauge was exceptionally generous height-wise.
My understanding is the locos were changed at Willesden. The tests were related to superheating as the LBSC Marsh I3 4-4-2T was a fuel efficient engine.
The LBSC also had a generous loading gauge (balloon stock was restriction 5 under SR days) and it may have been turntable lengths at the time which prevented other railways large locos from working through given the Marsh Atlantics were probably the LBSCs longest locomotives at a shade over 58' over buffers.