Why did the girlfriend cried on the train?

She was crying because, even though she loved him, she couldn't marry a black man. Remember it was in the 40s. Though he could pass for white, she couldn't be sure that their children could pass, which would subject them to the prejudices of the day. She also probably would have had a hard time explaining his blacker family to her family and friends.

P.S. I don't think his mother treated her badly when they met. she was shocked, for sure, but I think she was hospitable and polite, especially in the face of Steena chattering on out of nervousness and uncomfortableness.

I don't even blame her. He hates other black people but somehow expects whites not to feel the same way about him. It must suck to hate yourself so much.

I didn't get the feeling that the mother treated her badly, despite her obvious discomfort with the situation. It's easy to judge, but it would have been a shock for a young white girl, back then, who has no idea she is in love with a mixed raced man, and learns it so abruptly. Any young man in such a situation should have told her beforehand. Given some time to consider it, she might even have come to terms with it and accepted it.