He might've been deterred from committing the murder if he wasn't on the train at all but was instead in jail because he got immediately arrested by the cops for fare-jumping. Also murder isn't the only crime I don't want to happen on transit, that is most likely to be committed by someone who is already breaking the law against fare-jumping to be there at all.

so we should put fare-jumpers in jail because they could be murderers?

that's a strange argument.