All 4 axle diesels have pretty much run flawlessly from day 1 on the layout.
Early Atlas 6 axles ran well also but when I started getting the Fox Valley and Kato GE & SD70 Heritage units several track areas became serious issues.
1) One, the gap between the Westville and Brooklawn modules has been an issue from the early days (not the smoothest module transition).
2) When we did the whole house energy efficiency program it created heat problems (i.e. track expansion) in the loft. Evidently the high efficiency air conditioner ran less time which caused the loft to get hotter during the off cycles. The temperature difference between downstairs and the loft changed from a 7 degree difference to a 10-11 degree difference. Now I began to have problems on both ends of the viaduct (almost imperceptible unplanned S curves leading into the planned S curves). I didn’t notice the bad areas at first because the engines didn’t actual jam and come off the track until they reached the back of Camden curve (only accessible from the access hatch $%^&^$!).
3) When I added the switches to allow me to use the third track behind the viaduct as the 4th staging track, the track flow had a dip that caused the Fox Valley snowplows to snag on the rail joint and come to an abrupt stop. The track clearence on the snowplows is way too low.
All 4 areas have been ripped out and replaced/leveled. The new track has had the ties and rail sides painted, and been re-ballasted.
4) The layout has 55-56 switches and I try to clean them before every operating session. since most of my Peco turnouts are 40+ years old, this often requires filing and bending the point rails so they close smoothly. Before the last operating session I broke the points on one turnout. This turnout was always a touchy one, many times putting cars on the ground during switching, so I ripped it and the two tracks coming it off and replaced the turnout with another from my collection of spares. (I should have done it long ago!)
Now everything seems to be running smoothly (including the big 6 axles).
(When winter comes and the rails contract we will see what happens then!)