Here's why - the screenshots show what the problem was - huge seams/lines across the track which were impossible to ignore. Fortunately, advice from the UDK forums (as usual) offered solutions, though admittedly I did work it out on my own when waiting for a response. Here's a 'before' and 'after' example of the track lighting issues:
Lighting issues created lines across the track sections and hurt the game's visuals badly - they're finally gone after a whole day of mind-numbing clicking and exporting
This involved creating a second UV set on each section of road, using the automatic mapping to ensure that the UVs didn't overlap, and re-importing into the UDK. I had much more defined shadows initially but increasing the lightmap resolution caused UDK to crash and made the size of the level package enormous. I've had to scale them back down as a compromise which is a shame, but better to have a working level with general shadows than a broken level that won't start up or save.
I also intended to get the particle systems in-game tonight - so far i've got a water effect for the water towers in-game (which is the most important one).
I could do with a generic smoke particle for chimneys - this will probably be a modification of the steam car's smoke, but larger and slower.
I also finished the Maxton steamcar last night (after a last-minute catastrophe that almost looked like the car model was broken) - here's how it looks!
Tomorrow I have duties to attend to in University but I hope to get both the water 'power up' volumes and invisible health decrease volumes in the level by Tuesday night.