If you have the transformer set to full voltage and the engine is running at what appears as half speed we first need to know the voltage measured across the rails of the track when the train is running. Then we know if it is a transformer issue or an engine issue.
See if motor is getting hot with full throttle and only running at half speed. Reason I
ask is sounds like my 282. Doing exact thing and it turned out to be a bad armature.
My 282 motor got very hot. I would also make sure all 4 wires are in the prongs of the plug. I have a 336 that runs forward fine but half speed in reverse. If your 290 runs
half speed forwards and reverse then probably not the wires. I think my 336 has a bad wire.