After entering the train, she ended up at an Umbrella training facility (sometimes mistaken as the Mansion).
Rebecca and Billy battle the creatures and zombies in the facility and make their way out but before they can escape they are confonted by an enraged leech queen.
After the death of the leech queen and the destruction of the facility, Rebecca wants to check if some of her teammates, like Enrico, are still alive.
Since the mansion is near enough to the facility,she heads there after remembering that Enrico said that the Survivors of Bravo are heading there.
So while Billy goes his own way, she heads to the mansion.
She ends up at the residence where she takes a nap, and then encounters Richard Aiken, who escorts her to the mansion. They encounter Yawn, and that is when Richard is bitten, and why Rebecca is there with him when you encounter them as Chris.
Don't forget that Resident Evil begins AFTER Resident Evil 0.