OK, I worked it out.  Lever off (gently) the cover plate then undo the four little screws.  Then lift out the entire truck unit.  Then gently bend out the four phosphor bronze springs so they will actually touch the backs of the wheels when reassembled.  Of course there is a lot more to this story, I undid things I didn't need to then dropped things that I shouldn't have dropped, and broke wires I shouldn't have broken.  But it is _nearly_ back together

That's good advice, thanks Fred.  In desperation I started poking at the spring pick-ups with a craft knife blade, in between the backs of the wheels and the pick-ups.  That actually seemed to make one side of the truck pick up as it should (but not the other).  Given this Loco is the older of the two Whitcombs by a year or more, it's the one that ceteris paribus should need cleaning earlier.  I'll try your trick and report back.  I also have a surviving 0-4-0 gas engine that runs appallingly, so I'll try it on that one too.  Greetings and thanks from Melbourne, Oz.

OK, I give in.  More than a month and not a sausage    Surely _someone_ at Bachmann knows how to access and adjust (if necessary) the springy electrical pickups on the back of the wheels ?

I have two of these On30 Diesels, lovely models, heavy as bricks and originally both were very smooth runners.  Since then I have fitted sound cards to both, one of them runs beautifully, somewhere along the way with the other one, one of the bogies/trucks has decided not to pick up power, which makes the loco stoppy/starty, ditto the sound card.

I have run a multimeter over it, and it seems that the pickup(s) might be faulty on that bogie, but I have no idea really.

So my question is, are there instructions which would tell me how to access the springy thingies directly to check that they are contacting the wheels properly and/or remove the bogie to enable its replacement ?  I have the exploded parts diagram, but it doesn't really help in this respect.

I otherwise have no problem with removing the body shell to access the innards, but not the bogie.