Hi Kotangagirl,
To justify the cost of rostering a Garratt you need a long coal train (by UK steam era standards). York's yard were at the southern end of the ECML "race track" from Darlington but a tremendous amount of freight from Tees-side and East Durham joined the ECML via the Leeds Northern line at Northallerton. The line limit was 92 wagons (trains crossing the Tyne at Newcastle were limited to 56) and the usual practise for loose-coupled freight on the busy four-track section was to combine two trainloads behind one locomotive. Sometimes the combination in the down direction would go to Newport yard on Tees-side and at others they would go to Croft Yard at Darlington. Separating the train at Northallerton, where one half could go to Newport and another could go to Croft would not have been efficient, both in terms of line occupancy at the station and providing a second loco at Northallerton for the load going to Croft. The yard at York could easily assemble 92 wagons for the Garratt but I would not envy the task facing the driver in walking forward the loco to ease up the tension on 94 sets of three-link loose couplings and then drawing it all out of the sidings. The guard would no doubt berate the driver if he jolted the guard around and if a coupling broke, separating the train there would be consequences.