NJOGauge,
We recommend O54 due to the pilot on the ES44. The pilot "look" like they are fixed,, but actually pivot with a return spring that centers the pilot. The coupler itself pivots too. When running an ES44AC around O42 the coupler is stretched to its limits and will derail a car without a pivoting coupler, such as a 9950 die cast sprung truck. O54 curves relaxes these limitations, hence we "recommend" O54 curves, but the loco will actually ride around a O42 curve. That is what the catalog is trying to convey, with a lot less words.
Thanks,
Mike
Lionel