At last, I cracked it (thanks to the advice from ComradeP)! Just needed to invest in special steps for all my tanks and forget about capturing the second rail-city, which you don't need if you know you'll win within a couple of turns of running past it. Only managed it in turn 8 (230/500), but I think if I played it again I am sure I could do it in 7. I'm still no ComradeP
Does raise an interesting point about choosing extra equipment. It makes sense that this battle was not completable without them, but I'd got into the habit of not buying them. In the Russian campaign, I think I was left with over 500 prestige, since I was saving them up for when I _really_ needed them. I think this was the first battle I couldn't at least complete with my given units, which is what threw me. Probably lowered my end scores by not buying extra equipment, but I just want to horde
As a suggestion, perhaps prevent people being able to store more than 250 prestige (or that you have to spend it all in the next battle), so that they are encouraged to actually use it. The irony is that being able to customise your units makes the game a more interesting, while still allowing for generally historical orders of battle, but since I always want to keep it, I don't do it enough.