I agree with the other sentiments regarding the CCR. It's worth a brief visit but I wouldn't spend a whole day there. I'd be heading north to Glasgow on the first available flight.
In terms of trains south, LNER also operates from Inverness and Aberdeen to London King's Cross. The lines from Aberdeen and Inverness both have spectacular scenery, and the trip down the coast from Edinburgh to Newcastle is also stunning. Once you get south of York it's all a bit meh in my opinion. On the West Coast the trip from Edinburgh / Glasgow via Carlisle and Preston and on to London Euston passes through some fine countryside too although, in my opinion, once you get south of Preston it's time for a snooze...
Certainly the sleeper would also be an option - Fort William would be my preferred starting point - but most of the journey is at night and you'd miss a lot of the scenery. The new sleeper trains have had some teething problems but they are significantly better in terms of bedroom comfort and amenities (including en-suite bathrooms and double beds) than the previous Mk 3 coaching stock.
As for Amtrak in the US, I just travelled Chicago - Portland on the Empire Builder and it was a really enjoyable trip with a good crew, decent food, and an on-time arrival into PDX. Amtrak does a pretty good job all things considered.