(07 Apr 2021, 6:51 pm)L469 YVK wrote That was because the X21 was also running every 20 minutes and back then, it ran a quicker route missing Nedderton. So that (although it was still a good 10-15 mins more than the X20) plus the fact that the X20 was likely unreliable due to tight timings would've made the X21 a no brainer.
The X21 Nedderton section really doesn't have as much of an impact on people from Newbiggin using it as you seem to think. It adds literally 5 minutes onto the journey.
If reliability was an issue, which I'm not sure it was, surely the X19 would have the same problem? You'd still have to do Newcastle and back in less than 2 hours unless there was an interworking somewhere. Remember the allocation then was generally a couple of NCP2 Olympians or a B10BLE thrown in - both more capable than an E400.