one thing I have seen is corrosion in the center pin rails and you have to have 2 bad points before and after the effected track problem, sometimes you have to add jumper wires to fix the problem, you just solder a jumper on both sides of the rail due to a pin is corroded rail is open more then it should be to repair that kind of problem! sometimes you can run the train for a while and then shut down the engine and feel the rails with your hand if it feels warm anywhere on the rail then you definitely are having a resistance drop causing lower voltage in the zone or area of track!
Alan
