Zerogee
Clencher's Bogleman

As said above - twin decoders were never a good solution and rewiring with one newer decoder is a sensible idea if this is the case. Dud motors are relatively rare but are not too expensive, and pretty easy, to replace. I have heard of the Mallet having the front motor washed out with over zealous use of smoke oil, but never seen one, and as i just said - not a major problem anyway.
Quite. A loco with twin 55021s is probably OK, but a pair of the older 55020s or, as in my case, a 55020/55030 setup (the only one I've ever seen like that), is REALLY best replaced. As an example, I bought a first-generation Rugen 0-8-0 (the Mh53 green one) second-hand from another forum member a few years back, and it ran terribly, slowing to a crawl even on R3 curves - pulling the old (single) 55020 and replacing it with a Massoth L (which is basically like a 55021 but better and cheaper) made the world of difference, it's now a lovely runner.....
Jon.