Just finished a decoder installation in an Atlas GP7 (N scale). The decoder recommended was the NCE N12SR. It is, in fact, bigger than I would like. You have to mill away a significant portion of the frame in the short hood area to permit both it and the headlight LED to fit inside the shell. Even so, the LED body had to be ground down for clearance. 1/4W, 1K resistors were installed to limit current on the LEDs.

I would have liked to use the Lenz LE0511W or the new NCE Z14SR instead, along with a pre-milled frame from either Aztec or Southern Digital. Oddly enough, at the price they sell their frames, it's more cost-effective than using my own mill (though not as convenient).

One oddity was the refusal of the F0F/F0R to work as intended. The lamps would always come on opposite to the direction of travel, regardless on the NDOT setting in CV29. The decoder also refused to take a function remap of the WHITE and YELLOW leads using CV33 and CV34. I ended up swapping the WHITE and YELLOW leads at the decoder itself to straighten out the issue.

I will say that the NCE decoder makes for smooth running.