I asked about the couplers because if they are too still or ridged, they will pull cars off the track. I have a problem with my Lionel 0-6-0 dockside doing that all the time. I even replaced the coupler and spring, but still keeps pulling cars off of the track. Makes it a rather useless switcher.
But now that I think about it, it doesn't pull my heavier cars off of the track, so you may be able to solve it by adding weight to your cars.
For the locomotive issue, that's generally pickup roller spacing. I have several locomotives that died in the "yard" of my 1st O layout, before I knew any better. Sometimes you can solve that by rotating one of the rollers 180 degrees.
