an 0-6-0 steamer.  I have a coal tender and 3 cars.  It will only pull the tender and 2 cars at this time.  I just recleaned the track, all car trucks and engine wheels and still will only pull the 2 cars.

i am new to model trains, my train of only two years has started to slip.  It will no longer pull more than 2 cars around my flat track.  It slips in the same spots each time.  I have cleaned the track, slightly sanded the spots and after a few laps it starts slipping agian.  What do i try next?