(29 Oct 2018, 8:06 am)Dan wrote The 16:03 X1 service arrives at Easington Lane at 17:11 and runs light to the depot from there, arriving 17:33.

The peak flow of demand is Newcastle to Easington Lane on an evening, so the service provision is increased in this direction from every 12 minutes to every 10 (or more, with the X1As) and decreased to every 15 in the opposite direction.

It'd take over an hour for that bus to travel back to Newcastle, plus any layover, so if that bus didn't return to the depot when it does now, it'd be doing a service from Newcastle at around 18:30 by which time the evening peak has been and gone, and the number of people travelling is much fewer (to the point the service provision is reduced to every 15 minutes in this direction too).

Not wanting to split hairs, but if it takes an hour and 10 mins IN service from Newcastle, I'm not sure it would need to be allocated to a departure from Newcastle 1 hour and 20 mins after leaving Easington Lane when not in service. Particularly as it could use bus lanes approaching Heworth, wouldn't lose time around Gateshead Interchange, Prince Consort Road, Hills St etc.
The 921 doesn't take that long from Rainton Bridge and that's obviously in service too. 

However, the point still stands that the dead mileage even to Washington depot is quite high.
Appreciate the demand from Newcastle beyond 1800 may have dropped off a bit, but using the vehicle elsewhere, even for a part journey towards the depot, must be more beneficial than having it run completely light.
Exploring commercial opportunities and fitting it in to operational requirements is a challenge for any business. 
How many people would love a direct route from Washington to Dox International and back in the peaks? 
The 37 is often rammed and doesn't offer anything like the ticketing opportunities a GNE service has. 
Just one example off the top of my head to reduced dead mileage, whilst increasing revenues.