Rail is a long bar made of metal or wood that serves different purposes depending on where you use it. The rails on a staircase keep you from falling off the edge. Ships have rails around their decks so sailors don't tumble overboard in rough seas. A curtain rail holds your curtains in place above a window.

The most important rails might be the metal bars that trains ride on. These railroad tracks guide trains safely across thousands of miles. Two parallel rails form the path that train wheels follow, and they need to be perfectly aligned or trains could derail. Before railroads, moving heavy goods across land was slow and expensive. The invention of rail transportation in the 1800s transformed the world, making it possible to ship products quickly and cheaply over long distances.

To rail against something means to complain about it loudly and angrily, like a student railing against an unfair rule.

When something goes off the rails, it means a situation that was organized and controlled has become chaotic and disorderly, like a train jumping off its tracks.