Why do you think the women were moved to keep Lucius safe?

On the train to Lwow, a young mother, at great peril to herself, tries to protect Lucius from being arrested as an enemy sympathizer. The other women in the compartment do not betray him either. Why do you think they were moved to keep him safe?

Lucius was a kind, attractive young man that appealed to the women either as a husband or a son. The women gained nothing by saying anything, the young mother hoped that appearing to be a part of a family would protect all three of them

I agree with paulagb. Also, I'm sure everyone was sick of war and all the tragedies accompanying it. Lucius was on a personal mission, looking for a lost "friend", and not on military business. The women saw him as a regular person, someone who seemed as if he could have been a family member: a brother, husband, son, or a friend. Last time time the militias, loyal to Poland, came on the train, they detained all young men traveling alone. Likely there was very little civilians had control over those days. Maybe this was something they could do to help someone else.

One reason the young mother would protect him is that she handed her baby over to him when she thought she was going to an unknown fate and that way her baby would be saved by his fake father. Before the situation got that far, by pretending to be a couple, it would protect them both.
By this point in the war, Polish women would probably not want to call attention to anyone. They had seen too much of what might happen to anyone who was singled out.

Many women are attracted to men who seem vulnerable. His quietness and good manners led women to feel a connection with Lucius, and therefore they felt a need to protect him. It was not protection in the sense that he was weak, but in the sense that he was worthy of it.

They heard all the conversation between Lucius and Adelajda, the reason why he was traveling, they knew he was a doctor. The women could have betrayed them but they must have saw the goodness in Lucius, the way he examined Adelajda child which was sick, the way he held him and calmed him when the soldier took Adelajda.

I too think the women were war weary and perhaps also thinking about their own husbands, brothers, and sons and how they might hope someone would "save" them.