I think you could say something to them along the lines of hey, I know she's awful and we don't want to spend time with her, and I know its never pleasant having any kind of confrontation, but I'm really not comfortable being used as the reason why none of you spend time with her on the train anymore. I think she deserves to be told the truth as kindly as possible now because its not a good way to treat her, and no matter how unpleasant she is, she deserves more than these silly games you're playing at her expense.

That's the nice way. TBH I'd be going more along the lines of you all need to grow the hell up. If you can't do the decent thing and grow a spine and tell her how you feel, then you're worse than she is.