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.
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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 $%^&^$!).
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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!)