Have you thought about introducing some vertical slop for the rear drivers? (Make sure it is vertical-only slop!). Then fix a spring above and across the centre of the axle. The spring probably needs to be soft to avoid axle-wear and to allow vertical movement but strong enough to be a fulcrum; experiment! From the photo this looks as if it should be easy to fit. This means you will have a minimum of 4 wheels in contact with the rail as opposed to 3 now. This is the system Bachmann successfully use on many of their locos (57xx/8750 panniers, Collett Goods, N mogul, LMS Jinty, BR 4MT tank) to improve traction and pick-up; they spring the centre drivers. Good Luck!