Yep, out of quarter. I HATE plastic 'choo-choo's' for that reason. I re-quartered ALL my Bowser loco's and all my brass until someone 'borrowed' my Brazelton Auto-quarter and never returned it. Thats $175 I'll never get back. Of course that was 2003.
They were a nice kit. I built the one with a tender. MAKE SURE you have extra valve gear rivets as they wear out quickly on that loco. MOST faster than Bowser rivets.
I'd look at BRASS loco models too. For me I, like the OP, I want it as accurate as cheaply posible. I wanted all my articulated engines to pivot like the real ones. I built LOTS of Bowser kits and learned to modify their boilers and mechanics.
Even with heavily weighted 2-8-8-2's, getting 30 cars up a 2.5% grade is a task. I actually REMOVED the weights on all my hoppers and just switched to Jay-Bee wheelsets so the weight was at the bottom of the cars.
I would buy all the rights to the old Bowser HO scale steam engines and redesign the manufacturing process. 3D masters that can be scaled up to O and G scale. Corrected tenders. Corrected boilers with separate cabs. Can motors, NWSL gearboxes. New engines: N&W M class and 3 kinds of...
A really good book is John Armstrongs book "Track Planning for realistic Operation". Thats why I've used 24" and larger radius' and #6 and longer turnouts. I run N&W 1930's steam (USRA 2-8-8-2's etc).
I say its a problem with uneven and poorly layed track. My tiny little 2-8-0 HOn3 engines kept derailing and I soon discovered the track laying I did stunk. I went back to tracing radius with yard stick with holes drilled into it to correctly get those curves layed out. Easements and leveled...
20 years ago, when my skills were at the top, I did LOTS of model making, weathering and steam loco collecting. In 2007 I had a place to build my model RR. Lots of stuff changed and my skills have decreased along with a desire to finish the layout. Glad I never weathered the track or put down...