Its about...framerates. With my Toshiba Qosmio laptop (a couple years old), I can run pretty much any loco on any route with TSX on and High settings, and get 24-35 fps, no problem. In very dense yards, it drops into the 18-20 range, which is still useable, but not great. However, I've been using non-TSX mode with the Consolidation, which is really tough (dare I say...impossible?) because the cab is so dark with TSX, its pitch black without, even with the brightness cranked up.
In the extended scenario, I was getting 18 fps when it started with TSX on, but soon after leaving the initial position and entering the yard, frame rates dropped into the single digits. Which is clearly not useable. So I tried every combination of turning things off in the graphics settings, eventually finding that turning TSX off instantly gave me 40 fps. So there's something about just this loco, combined with TSX, combined with my laptop that makes for a bad situation.
Pretty sure its the fact that its so beautifully detailed and geometrically complex, and the Train Sim gods dont want me to experience it in all its glory on my old laptop. So...is there anything I can try to reduce its impact, or is this just a poignant hint that it may be time to start the long process of saving up for a new machine yet again?
That's exactly what I was going to say. It's not the loco, it's those yard lamps in Woodvale yard. I found that when you get there, if you simply look over to the other side of the river, away from the lamps, the fps rocket up again. I think it has something to do with the way they did the light emitters for the lamps. It's kind of a pity that I chose to do a scenario there 
