When they discuss which is the correct lever to break the train, why does the girl pretend that she's crying?

Hero Girl's "issue" (they all had one to deal with) was that she didn't believe in herself. She told Hero Boy (stupid names for characters) the correct lever to pull, but when he said that another lever looked more like a proper brake and she must be wrong, she backed off when she should have held her ground. Then when it looked like he was going to pull what she knew was the wrong lever, she covered her face because she didn't know what would happen when he pulled the wrong lever.

When HB sees HG covering her face, he considers the possibility that she was right the first time and goes back to pull the lever she told him to.