A rail is a horizontal bar attached to posts or fixed round the edge of something as a fence or support.

A rail is a horizontal bar that you hang things on.

Rails are the steel bars which trains run on.

If you travel or send something by rail, you travel or send it on a train.

If you rail against something, you criticize it loudly and angrily.

If something is back on the rails, it is beginning to be successful again after a period when it almost failed.

If someone goes off the rails, they start to behave in a way that other people think is unacceptable or very strange, for example they start taking drugs or breaking the law.