I've even found variation between RTR locos of the same class. Back in lockdown 1, out of sheer boredom I tested how many of my coaches (16 at the time I think, a mixture of RTR Gresleys and MK1s) each of my locos could manage around my roundy-roundy. I found one of my Hornby A4s was much more sure-footed than the others, easily managing all 16 while the others either couldn't or slipped a lot. I never got to the bottom of why...