If my memory is correct, the V4 was faster and more stable than the K4 on the level stretches of the West Highland Line but couldn't match the K4's adhesion on the hills and bends. Pros and cons of a trailing truck I suppose. A1SLT certainly score highly for forward planning and I think a fair amount of design work has already been completed for the V4. It will be very interesting to see if they can make improvements to the Gresley conjugated gear. Before that though, there's the matter of the Lentz-Franklin gear on the P2! There may be a few tests and challenges to get that finally sorted out. Any bets on a completion date for the V4? My guess is 2032ish. I think we may have thinned out a bit by then.