That was a very comprehensive video. I noticed some railwaymen's jargon crept in. Do US railways have 'consists' and 'diagrams', I wonder?

Joe Offer wrote: Seems to me that there's a difference in the connecting rods between US and UK steam locomotives, but I'm not exactly sure what that difference is. The connecting rods on UK locomotives seems to be continuously horizontal, and the US connecting rods look different. What's the difference, and why?

I don't know about US locos in general, but take a look at this photo of Big Boy from the US National Railroad Museum website.

The sloping bar is the connecting rod and drives the third wheel back. In common with all (?) locos there is one or more horizontal coupling rods joining all four wheels, but you can barely see most of it behind other gear.

I think on most British locomotives the coupling rods are more visible. See this photo for example - though in that case it's stopped with the conecting rods and the coupling rods all lined up.