I don't see any ACL, unless there's more pictures than the four you linked to in your post. The trailing locomotive and both passenger cars appear to be all SAL. The trailing locomotive is No. 17xx, which is in the SAL's road number series for GP7s. If I remember correctly, ACL's GP7s were numbered in the three-digit 100- and 200-series. Also, although the photo is black-and-white, it's clear that it's the darker color of the SAL with no ACL stripes. The man onboard is concealing the SAL stripe. The distant passenger car has a letterboard that appears too short for "Atlantic Coast Line," but "Seaboard" would fit.
It would be interesting to find out how and why the NS GP18 was used (the logical is a mechanical malfunction on the SAL GP7).
DFF