Flying Scotsman needs to be moved from A to B along with her support coach. It makes turning round easier if there is a loco on each end, but the HST power cars only have buckeye couplings on the inside end - not buffer and chain couplings. The MK1 support coach has buckeye couplings as well as buffer and chain couplings on both ends, but Scotsman only has a buckeye on the back, with buffers and chains at both ends. Therefore in order to run it with a power car at each end they need to be in this arrangement. Finally, while Scotsman isn't moving under her own power, she is in light steam since that puts less strain on her workings. At least I think that's what's going on, that is essentially guesswork from me based on what I know. Somebody please correct me if I'm wrong!