Do the scale Dash 8 and ES44AC really need 42" radius curves?
VERY IMPORTANT! Radius is equivalent of O-84 in 3-rail speak. I often see people (and magazine article authors) refer to O-72 as a radius; it's a diameter (twice the radius.) Not to be a picker of nits, but being aware of this is the difference between a nice operating locomotive and a shelf piece or eBay sale.
That said, so far ALL of my 6-axle MTH diesels have been able to negotiate 36" radius (O-72) curves. The locomotives tested so far include the CW44-9, the CW40-8, C40-8, SD70ACe, and ES44AC (and Hybrid with test car.) Would have bet money the SD70ACe wouldn't have done it. You must have "flat top" rail -- Atlas, ScaleTrax, RealTrax, Gargraves, Ross and [maybe] FasTrack. Your track work must be free of odd dips and twists and kinks as these locomotives WILL find them. That has been my experience at the club, especially when locomotives are MU'd as there is slippage/pushing that naturally occurs. If your track is a little rough, replacing the scale wheels with Hi-rail wheels will solve the problem (I haven't bought any yet, though.) One caveat: these engines are not fond of curve-replacement (i.e.-O-72, etc.) turnouts when using scale wheels. They seem to like #5's. They will also despise reverse-curves ("S" curves) without 20" of straight in between.
Also, I use Kadee 805 couplers in my locomotives as opposed to the 806's. The longer shank allows the coupler to swing more and allow for tighter radius with cars coupled. The way to fit them is pretty simple -- just shave about 1/8" off the rear of the coupler box (just past the rear mounting hole) and the truck will clear the box. The shank doesn't extend past the cut so that's not a problem. The overall performance improvement is worth it.
