I had already been the "guest of honor" at at gangbang more than once before I first heard the phrase "pulling a train".
It was uttered by this very sweet older man dressed head to toe in black leather. He approached me at party, took my hand in his, and in his soft (Georgia, I think) accent congratulated me on "pulling a wonderful train" the previous party...
(I sometimes miss New Orleans so...)
Anyway, I looked at him with my best intelligent blank look and responded "I beg your pardon, Sir?, (he was a DOM, so the Sir was obligatory).
I will repeat his answer here as closely as I can recall. It made quite an impression on me.
"My dear, in my time, that phrase was used to describe the event whereupon a young lady such as yourself willingly participated in serial intercourse. It was commonly referred to as "pulling a train, the young woman in question acting as the "engineer" followed by a long row of eager "cars", if you will. So much nicer than "gangbang" I think. Besides, in uncouth company a girl might be forced to participate in a gangbang, so "pulling a train" implies willing participation, I think. But times and language change, so I suppose now days the phrases are quite interchangeable". He said some other nice things about my scene, my body and my sexual responsiveness, spoke privately at length to Leigh, thanked me again, and went on his way.
A word of explanation; it had been a BDSM event, I had been blindfolded and fastened to a "horse" and so did not actually see all those who participated in the scene the previous weekend, so I really didn't remember him, or what part he took.
I have heard both phrases used interchangeably since. The biggest difference is "Are you pulling a train this evening?" seems to be preferred to "Are you the girlie gettin' gangbanged tonight?"
I have never at anytime heard either phrase used in a way to describe how many partners (or orifices, I guess) were involved at a given moment. Somes times a gangbang (or train) might be limited to simple "serial" vaginal intercourse, or involve simultaneous oral/vaginal penatration (I still don't like the phrase "spitroast") even triple (oral, vaginal, anal) penetrations, or scenes where there are so many things happening at once it is past sensory overload.
(Imagine a woman surrounded by people, Women as well as men, with multiple penetrations, soft, hot wet mouths on your body everywhere, and hands, too many to count, stroking, probing, touching you all over, massaging various bodily fluids into your skin... it can be VERY overwhelming.)
*AHEM*
Honestly, I use "gangbang" out of habit, I suppose, first learned and all.
And no offense to anyone here, but I think the "Urban Dictionary" is mysoginistic crap, mostly.
WW