I had a few similar problems, locos with traction tyres get bad wheel slip, I replaced the tyres with a paint on stuff from the US called bullfrog snot, don't know where they came up with the name, but it works, my radius is 2 and 3, i replaced all of my couplers with knuckle couplings, coaches with sprung buffers had the buffers retracted and held back with a spot of super glue, had issues with the park royals, had to make a minor mod to the bogies, they work fine now, bubbles etc, no problem, anything longer than that without bogies is a issue, six wheel coaches, forget about it. I did have issues with Murphy's 071 but it seems to have bedded itself in now and is running fine, 121s 141's 181's run great, 601 runs alright with five wagons so long as the track is good and clean. Speaking of cleaning, that's a problem, solved by buying two lilliput track cleaning tankers on fleabay, I panted them up yellow and disguise them as part of a weed train.