Others may disagree, but in my experience ANY Atlas engine with just two pickup rollers is going to have problems stalling on switches. Fortunately the U23b has cutouts on the trucks for mounting two more pickup rollers like they use on their Masterline engines. My U23b had the same problem you describe, but I had a SD35 dummy engine that came with four pickup rollers. I removed the inner rollers and their hardware. Mounted them on the U23b trucks and wired them up. Problem solved, the engine is a very smooth runner with no power pickup issues. Perhaps you can check with Atlas to see if they have the parts.
I have an early Atlas RS-1 that came with just two pickup rollers and unless the track and pickups were pristine clean it wouldn't hardly run. The trucks did not have the modification necessary to mount the extra rollers so I fortunately found a junker later issue RS-1 with four pickup rollers. Transplanted the trucks and now my RS-1 runs like a champ. No stalling problems at all.
I've had the same problem with their Trainman RSD-7 (which is a shelf queen because of this issue) and their early SD-40. In the SD-40 case I have the powered engine tethered to a dummy to get the four pickup rollers. Engine runs fine now.
Ken
