While playing as Billy during the mission to apply the breaks of the train will likely yield no emotions from the player, playing said mission as Rebecca will feature a cutscene of her encountering the now zombified Edward Dewey who was the helicopter pilot that transported Bravo team to investigate the murders. She is needless to say greatly distraught by the situation and futility does all she can to avoid the inevitable that she must kill her teammate.
It's a small moment, but in the habitations/save room in the Treatment Plant stage of Resident Evil 0, if you check all the bunks, on one you'll find a photo stand. Written in the bottom corner of the photo is the sentence: "Daddy, please come home soon." Reminds you that even people who work for the Umbrella Corporation have families.
And then the Fridge Horror kicks in as you realize where this worker's family likely lives...
Rebecca Chambers's last meeting with Enrico, her team captain becomes this in retrospect. Had she gone with him, perhaps he'd have stood a better chance at survival and not have been killed by Wesker, but then again there was little Chris Redfield and Jill Valentine could do about that too.
Rebecca and Billy parting ways. Despite their rocky trip throughout most of the game and the circumstances behind their original positions the two came to greatly respect one another and you can tell that it's difficult for Rebecca to let him go on his way whilst she goes on to endure another night of fresh hell. There's some comfort to be found in that Rebecca goes on to write a report successfully helping Billy to fake his death so he can have a shot at a new life.