In OO I would start by seeing what the loco on its' driven wheels alone can do by way of haulage, and assuming this is OK, then replace one set of loco carrying wheels at a time, then the tender, testing with each addition, to see exactly where the problem lies. Tends to be one of two things: carrying wheels taking too much loco weight or even part lifting the loco, one or more very draggy unpowered wheelsets. Hope this is of some help.
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