I was fortunate to get the S1 and Q2. Both have been run on my Atlas track with few problems. My curves are for the most part O-72 and greater except for an O-63 reverse loop and an inside track on a 2 tracked trestle which is also O-63. I have not run these engines on these curves.
I think that larger engines such as these need excellent track work to operate flawlessly. Initially,I did have a problem with the S1 at a homosote joint that would give due to the weight of the engine. Once this spot was bolstered and track spacings were optomized the engine ran quite well.
I can say the same of the Q1 and J1a.
Norm Rish