I wonder how many locos were involved in the WW1 Naval Specials to Scotland: Euston-Thurso (717 miles daily - at least 6; from Euston, changes at Crewe, Carlisle, Edinburgh, Perth and Inverness - possibly more?), or Devonport-Invergordon (721 miles in 24hrs45min - via the Severn Tunnel and the North&West line, including meal (& presumably loco change) stops at Taunton, Shrewsbury and Perth: other loco changes I would suggest at Crewe, Carlisle and Inverness, and possibly elsewhere). To which also allow for double heading, bankers, etc... - so into double figures, at least at times.
Or the Naval Grand Fleet coal specials: Pontypool Road-Grangemouth, various routes, about 350miles (or more - some of the routes were a bit devious), taking about 48hours...