(8 hours ago)Nerd4321 wrote Im assuming its when Arriva wanted them all to be normal
I remember it used to be
X4/X5/X6(Short lived Cobalt), X13,X10/X11 & 10/11
Im assuming once the X6 was withdrawn they wanted them all numbered together
X4 to X7
X5 to X8
X13 to X9
X10 & X11
That X6 didn't exist with the X4/X5.
The original X6, was a variant of the X5 in Blyth they used to do different routes in Cowpen. Then it was the now 354 but via South Gosforth when they had their complete moment of madness where the X4 went via Gosforth and the X5 went via the Freeman which was very quickly changed into the 52/54 as it was extremely unpopular - as an understatement.
Can't confirm but something in the back of my head says it's because they didn't want the MAX/Sapphire routes to duplicate in numbers and the X4 already existed in East Cleveland though.