(06 Jun 2016, 6:25 pm)Andreos1 wrote I don't think they ever have. Too many have been and gone over the years (like the X20 - and X3 before; X95/96, the coalfields - Newcastle via Sunderland expresses and the more recent X3 up Newcastle Road).
With regard to the X20, I don't think there was enough variation compared to the 20. It wasn't that much quicker and didn't deviate too much from the existing route.
When you look at the 21 and X21, the difference is/was noticable and whilst the 20 can suffer with reliability, I am not sure it was ever to the extent the 21 can suffer.
It was the Durham to Houghton section that was quicker. I used it every afternoon from work when it was introduced, and you did save massively on the 20. It would do limited stop to the Belmont Sportsman arms, and then non-stop to Houghton, iirc? Perhaps it served West Rainton, but that may have been the 20X!
Sunderland suffers because the city centre is a mess. It's gone from being a thriving high street, to a pretty non-existent high street, and a shopping mall full of pound shops or cash for gold places. Binns is obviously long gone too.