What I think may have happened is that the Royal Scotsman train has arrived, quite late in the day, not from the south but from the north. As it is so late in the day (sunset is around 7 p.m. in Scotland just now), I would expect the passengers to be detrained in the main line up platform (Platform 2?). There are several good large hotels in Aviemore where they could dine and maybe stay overnight. Also, in my experience, railways are never keen on leaving passengers inside a loco-hauled train during a propelling move. The engine that has brought the train from Inverness would then draw it down, and propel it back onto the Strathspey line. For some reason they seem to propelled it further onto the Strathspey line than is usual. I think that is the reason why Flying Scotsman is going so fast. The driver doesn't expect to reach the train for another hundred yards or so.
When working as a goods guard in the '70s I often acted as secondman on diesels running light, especially when returning to Eastfield shed at the end of our shift, so I have some sympathy with the fireman here. I especially remember one occasion where I said the wrong thing. I reckon we must have worked a train finishing at Cadder, and we were coming into Eastfield from the east. I can't remember now who the driver was, but it was one I new well and we were chatting together. We stopped where we always did, in the second loop, just inside the cat's eye signal that would take us into the shed. We both opened the cabside windows and looked back in the direction we were going to go. The signal was on my side, and it was still on. I casually said to the driver, "That's us," meaning "We're finished for the day, or we will be in a minute when we get the engine into the fuel road." As soon as I said it, I heard the driver releasing the direct air brake and starting to open the throttle. He had taken me to mean "That's us, we've got the road," which is what we trainmen usually meant when we said "That's us". I quickly called, "No! The signal's still on," and the driver shut the throttle again. A second later the signal did clear and we set back into the depot. After 50 yards or so, we cleared the hand points for the fuel road, so I got off, pulled the points, and the driver drove into the fuel road. I probably let him drive the last few yards himself, so I'd say goodbye after I pulled the points. I'd then start to walk home as I lived near the sheds. My driver probably knew how stupid I'd been, but was too polite to say anything. But I knew, and the fact I clearly remember the incident 45 years later says it all! Since we didn't actually pass the signal at danger, no harm was done. But it does show how easy it is to make a mistake.
A contributory cause in the present case is the design of that corridor tender, which so effectively blocks the driver and fireman's view when running tender first. Who can we blame for that? A certain Nigel Gresley perhaps? But there's probably no other way to carry enough water and coal, and to fit in a corridor too, but to build the tender right to the loading gauge. Of course you don't need all that water and coal capacity to go from Aviemore to Broomhill and back, but that's not what Flying Scotsman was designed for!!