Railway lines also use flags. Railway flgs tell the trains what to do. for example, a red flag tells the train to stop, just a stop sing. A blue, white, or green flag tells the train that it can go. A blue flag tells the train to stop, just like a stop sing. A blue, white, or green flag tells the train that it can go. a blue flag on the side of a train means someone is working on the train. It means nobody can move the train