Tees Valley, you can feel the desperation pouring out the sky as it sets over the boro, try 6.30pm october with no clouds
I can remember watching youtube videos, I think it was the 'British Ace' and he used to say the best sunsets/rises he had seen in game were on the GWE.
Just don't enable fog as you get a kind of dreary overcast at night/pre-dawn (with no fog!) which then just transitions into the fog effect.
SEHS, I think the last service in the second journey chapter, 395 Faversham to St Pancras, very nice sunset.
One of my all-time favorite scenarios on TSW is "Suburban Sunset" which you can find on the Long Island Railroad route. It's essentially an early evening run on the mainline heading westbound on a stopping service until Jamiaca station, which then proceeds as an express service all the way to New York Penn. Along the whole journey you'll be pretty much chasing after the sun, and if you're quick enough you might get held up around Harold Interlocking to let some trains pass ahead of you. It's picturesque and does a decent job at capturing what the route is all about.