This model's always been troublesome. I sent it away to be repaired once and it worked again for a while. When it seized up a second time I gave up on it.
With a central motor driving both bogies, electrical pick-up from all wheels, lots of weight at both ends and a 5-pole motor (one of Farish's earliest ones as I remember) it ought to run beautifully.
As I take it apart and check the components the first obvious thing is the gearwheel on one of the shafts.
The teeth are almost triangular from wear on one side. This is the shaft that keeps seizing up. I put it back together without this shaft, but with just one driving bogie it only moves happily in one direction.
By the time I've forced the bogies out and back in a couple of times the pick-up wipers are bent out of shape. I need a more reliable method of current collection.
I solder this thin flexible wire to the pick-ups on each side of each bogie.
One wire is connected to the metal strip under the chassis, the other to the clip that keeps the carbon brush in place (I couldn't get the solder to take on the heavy steel of the chassis). At least all wheels were now collecting current.
Even with the drive shaft removed the motor was still getting stuck. Eventually I realised that the circular parts around the motor could move, and were pressing in on the motor preventing it from turning.
With the flat part in a vice I forced the circular section out by about a millimetre.
Solved that one, and suddenly I saw what was in front of me all the time. The gearwheel teeth were wearing away on one side because the gears weren't aligned. Pushing the corresponding wheel back along the motor axle gave a much better mesh.
I've always liked this model, and its great to have it running again, hopefully for longer this time.