Thanks all for the advice on this problem.
I have spare smoke unit fan motors but they are all Lionel and date from before Lionel got in what is supposed to be a more reliable batch of these parts earlier this year. So I have taken this loco apart and given the motor a shot of 9V from a battery and some light oil on the base of the driveshaft and its bushing - I followed Eric Siegel's video tutorial on fixing noisy Lionel fan motors. It's a fiddly job but not that complicated.
This has restored normal operation and I'll see how long it lasts. I'm not optimistic but it will do until I can get either MTH motors or some new ones from Lionel.
I can confirm that before I did this the motor would either spin briefly on startup before the 3 blinks started or not spin at all followed by the blinks. Interestingly applying battery power to the motor also caused the marker lights to illuminate. (The motor wires are soldered to the top of the smoke unit and disconnecting them was not an option.)
