Thanks for the latest comments. I'm in the process of sealing the railway up for the winter but the Indian Summer weather (ending tonight) has given me a chance to test a bit more.
First I tried to a good clean up of the point blade. Used the track cleaning fluid and toothbrushes. But no effect. Rugen still stopped. However, then I discovered that when I pushed the manual lever of the point so that the blade was hard against the rail, the Rugen took off, just as it did when I used the voltmeter. I've also noticed that the point (and signal) motors are not well supported from underneath right now. Because the railway sits on a soft earth bed on a raised wall, gentle erosion is a constant problem. So I guess all of this, and the note about it being a heavy machine, might explain why the Rugen has a problem there.
However I'm glad that finally somebody (
@LVT) has come forward with the same experience of the Rugen as me. From your post I understand that this model has always been without skates, and probably always will be, is that correct? The wheelbase too short to fit a skate? (My model is a 28003, bought new about 8 years ago). My railway does usually have a strong German coast theme, including a rake of three Zenner Rugen coaches, and I do like the look of the machine. On the other hand I never actually saw one in action on Rugen - it has always been the big 2-10-2 beasts when I've been there - and it looks like locos without skates may be something I should avoid. I'd rather sell a loco than just have it sit unused; at least its good to know that if I do sell it, I'm not selling damaged goods.