(02 May 2022, 10:11 am)mb134 wrote The Ashington express routes (bar the X21/22) are arguably much more demanding than the GNE X21 - the X14 up at Thropton is very hilly, the X15 is constant foot down on the A1, the X18 is rural NSL roads virtually the entire way combined with A1 flogging north of Belford, and the X20 is a combination of the X15 and X18 in that it's constant foot down for the southern section and NSL country roads north of Ashington. 

Aside from this being more demanding than the GNE X21, there was no B5TL available when they first started gaining E400s and at this point fleet standardisation is probably reasonably important.