Huh. My "real life" (if not always trustworthy) memory of what ran on the M&E only dates to the mid-60s, so it seems South Orange trains had expired by then. But oddly enough, if you look at the DL&W entry in the 1952 OGR
(the only thing online I can currently point to), you'll see there are no trains that either originate or terminate in South Orange. I'm not sure why there would be South Orange trains 10 years earlier in '42, none in '52, and then South Orange trains again three years later in '55.
The train numbering you recall is uncanny. From the '52 OGR, it looks right, but again, no 200s for South Orange (or 300s for Summit). Also there were 900s for Dover and 800s for Gladstone. I'll have to check the mid-60s OGR I have elsewhere to refresh my "real life" memory.