Firstly, KX to YK is too far for TSW. Secondly, Virgin won't licence their brand anymore (TS20xx had branding pulled). Thirdly, suggestions should be in the suggestions forum and should include details such as rolling stock, stations, operators and distances.
LNER would be the go-to for the ECML, surely? As for stretches of it that would work in TSW2, you'd be more likely to see something like Newcastle-Darlington where they have existing assets to start from when it comes to the station at one end of the line, you have the INCREDIBLY scenic Durham viaduct in the middle, and there's the fun loop in Newcastle and Gateshead where you come in across different bridges depending on which end of the station you're using. Great opportunities for either LNER 801s and Transpennine 802s, or 225s and 185s respectively, depending on the precise setting. (And hopefully a mod will move this thread where it belongs)
I take it you mean the 156, the 150 doesn't and didn't run on the WHL - and left Scotland in 2003 (iirc). Also, it's likely either the 156 or 37, the overlap is a very short period.
That is too far 175 mi 280 kms the game engine couldn't handle that route. Shorter by 32 km 20 mi than the Westbahn ÖBB Salzburg Hbf Wien Hbf 195 mi 312 km with the freight yards being Salzburg Gignl and Wien Kledering.
The game engine could almost certainly handle it, it's just that a route of this length is far too long in terms of development time, especially with the amount of stations and scenery on this line, in particular. Also, I'm going to give this a 10/10 for hypocracy and self-promotion of a suggestion. Saying this route is far too long for the game engine (as in, it can't handle it), and then immediately saying '20 miles less than my suggestion, which has these two freight yards' both things are completely irrelevant to this thread, by every measurement.