2x Optare Solos have been on X66/X67 today.
It seems a regular occurrence for Solos on the Darlington MAX services, although with 4645/4648/4655 being kept local and 4651 on long term VOR it's quite clear why those services have the rare allocations.
It would make sense to allocate either the Gas Buses or the Wright StreetLites as suitable spares, rather than use the Optare Solos which end up overcrowded when I've seen them...
Arriva North East: Rare & Odd Workings - December 2019