I was taught that the DD's were used to pull pax trains from LIC and Penn into Jamaica, go into 4 or 5 track, cut off the DD into an E yard track, and then put a steam eng onto that train from a adjacent E yard track. This was the purpose of the E yard tracks, which originally had 3 tracks. Westbounds would cut the Steam eng from 1-5 tracks and the DD would back onto the drag from anywhere with third rail, 0, the Atlantics, etc. The interlocking machines at Jay and Hall had a special traffic feature in the station tracks to facilitate these moves. Remember, this was in the era when electrification extended only to Mineola, then Hicksville. They would also stack trains, or add "sections". For example, they'd take a train out of Penn say for Montauk, cut the DD at Hall's end as described above, bring another steam engine with several cars behind it from Jay, put Greenport people on that and run the whole thing east on the Mainline. When the train would get to Manorville, the first part would separate and go down the track to Eastport, and then the second section would continue east to Greenport. I've heard they brought up cars from Brooklyn in this manner, including supposedly August Belmont's private car which had it's own track in the old station. These electric engines also did freight duty, and at one time third rail ran down the Lower Montauk Jay to Pond for this. I don't know what type of electric engine, DD or whatever, be damned if I could remember...
