With a woman that enjoys gang bang or trains, whether it's something she's currently active doing or just a one time thing that happened years ago?
In our swinging, I've seen my wife do many a thing. However, I've never seen her do more than 2 men. Usually I was one of the two. She bi and at parties, she seemed to enjoy the other ladies more than the men at the festivities. I would have let her, had she requested or fantasized about it. She sure let me give in to my own kinky perversions with others, so yes, if she wants to pull a train, hell yeah!
All depends on how they are willing to have it play into our relationship. I'm not really opposed to anything short of BDSM (might dabble a bit in the BD, but no interest in S&M) but I also think that anything you do in a relationship affects both of you so I'm not going to give carte blanche to a partner. They have to be willing to make their desires mesh with mine if we are truly in relationship. And if it was something they did years ago when I wasn't in the picture and they aren't doing it anymore, then I really don't care. I'd like know it because it sheds some light on them, but it's past and part of their history.