I've changed the plan for the small goods train at Clanfield. It now has a class 08 shunter with DCC sound and magnetic couplings; and will be pulled by a loco with magnetic couplings.
We've created a new schedule for goods "running around": it will exit Clanfield to a storage track, periodically run around back to the storage track and eventually return to Clanfield.
The loco should be uncoupled on arrival; then the shunter needs to come out and claim its wagons to go to the factory.
For the train to depart: the loco needs to move to a different platform; the shunter needs to return the wagons to platform 1; then the loco needs to be manoeuvred in to join to the wagons.
Unfortunately the new class 31 loco will not train track from the station "throat" area into platform 1 if the wagons are present. It will track into the platform if the wagons are absent. It will train track when running around the railway in either direction. If it is reversed onto the small goods train at the contained yard, it tracks and joints on correctly. A different (class 58) loco DOES train track int the wagons at Clanfield platform 1.
We cannot work out why this is. We cannot see any properties on the wagons or the platform that suggest why the class 31 won't work. Train length was a possibly issue (the combined length of the goods train + loco exceeds the block length) but this problem occurs with a minimal length goods wagon set.