So I have a E6A that runs fine on my small 2' x 4' with the outside track being 11" radius and inside 9 3/4". Shouldn't the Kato SD70M also run on the outer 11" radius track? The E6A runs fine on the 11" radius NOT the 9 3/4 although I think it might, but look weird. Thanks! DJ
Sometimes you can finesse equipment to run on tighter curves by easing the transitions into & out of the tight curve from/to the straight track. Use a section of longer radius at the entrance and exit from the curve. You may have to use a section or two of even tighter radius in the middle of the curve, if the original curve was a tight fit in its available space. For example, an eased, 180 degree curve might be 12.38r45, 11r45x2, 12.38r45. Another example of an eased 180 degree curve could be 12.38R15, 11r45x3, 11r15, 12.38R15, which would be a little more compact in span. Similarly, the corresponding inside eased curves would be 11r45, 9.75r45x2, 11r45 for the first eased curve above, and 11r15, 9.75r45x3, 9.75r15, 11r15 for the second. How good the equipment looks while successfully negotiating the tight curve is a different matter, but it's better looking than a derailment!
KATO 6-axles including a ES44AC and SD70ACe should work with their sharpest curve radius that Unitrack has. As long as you keep the standard long-shank couplers on, they can navigate them really easily and not look "too" silly as well. The long-shanks of the KATO couplers also help navigate the first car behind the locomotive very well without derailing (for truck-mounted couplers). However, if you upgrade with Micro-trains couplers, make sure the first car behind the locomotive is long-shanked as well or body mounted couplers. Here is a quote off Kato's site: "The curve on which the model is designed and developed using the factory-equipped couplers, is a UNITRACK 249mm (9-3/4”) ground-level curve for N scale models." From my experience, the coupler length is very important and shorter couplers on the engine or cars can pull off either the engine or the car. Also curves or turnouts at the bottom of gradients can be deadly.
Oh, in answer to your original question, how does the SD70M compare to the E6A in terms of the distance between the truck pivots? How about how far each truck can swing without hitting the shell, etc.? Finally, while the loco may make it through the curve by itself, depending on how much over-hang from the truck pivot to the coupler, it may pull the lead car off the track. I was under the impression that all Kato N scale equipment could negotiate 9.75R curves (again, the hardest part is often the entrance and exit from/to straight track, particularly WRT coupled locos/railcars)
Beware that 9.75R is not the smallest radius N scale Unitrack curve that Kato sells any more: they offer 8-9/16" radius Unitrack curves now. And even smaller radii (down to 6") are available in their Compact Unitrack series.
Those are the curves I run on my layout and I don't dare run anything larger than sort 3 axle diesel on it or very small drivered 3 axle steam. But the big Kato stuff will run on 9.75 but may not look good doing it.
I guess I should have said I was using Atlas code 80 track pieces 11", 9 3/4 ", and straight pieces going into the curves? If it makes a difference. My DC E6A with old style couplers works fine on the 11" radius, and even the 9 3/4.
I have a few Kato SD70MAC that perform flawlessly on 9-3/4 Atlas Code 80 snap tracks as well as Atlas Code 80 flex track curved to 9-3/4. I believe the SD70MAC has the same chassis as your SD70M.
