Out of interest, why is this a prerequisite of you visiting somewhere? Northallerton already has a number of non JDW pubs you could go to instead.
If local rumour is to be believed, the now vacated police station on the High Street could well become a Wetherspoons in the fullness of time.
Apparently the new pub on the High Street (The Potting Shed?) was originally going to become a Wetherspoons, but the delays in the NHS selling the site put paid to that (it was the former Rutson Hospital).
Travelodge have apparently also taken an interest in the police station site which could tie in with the Wetherspoons development (rather like the Posterngate development in York).
With regards to numbers etc, as well as both the District and County Council, North Yorkshire Police have recently moved their headquarters into the town centre (in the old Rural Payments Agency/Defra/MAFF building). I know some of their staff who now travel into work by train (vice travelling to the former Headquarters in Newby Wiske village by car).
A lot of people from Catterick Garrison/Richmond/the Dales do travel south on the ECML from Northallerton (car parking is cheaper at Northallerton than Darlington apparently) and the distances to/from the stations are roughly the same. I even know some people who drive north from near Thirsk to then travel south on the ECML due to Northallerton's more frequent direct train services.