FWIW, I've used ZFV-EWR (and back) extensively, and United seems to think (almost 100% of the time) that I didn't use the ticket. I know this because my return ticket often has the red letters of death on it online (that the ticket needs to be reissued and to call them about it). I call, they ask if I took the train, I say yes, they fix it. But it seems pretty weird how they almost never get the message that I'm taking the train, given that I always do.