The larger railroads, and some of the small ones, are unionized and they work on seniority. The guys who have been there longer have first choice on the jobs. New hires usually spend a lot of time on the "extra list" or "spare board" where you work jobs as they become available, regardless of time of day or day of the week. After you've been there for a long time you might be able to count on a set schedule but for the first few years forget it.
Some short lines are non-union, and some of them only run daytimes and weekdays. So that's a plus - on the minus side they usually pay less than the big railroads. Sometimes a little less, sometimes a lot less.
It's your choice.