Rail has multiple meanings depending on the context. Here are some of the most common definitions:

• Rail (noun): A bar extending from one post or support to another and serving as a guard or barrier, a structural member or support, a light structure serving as a guard at the outer edge of a ships deck, a fence bounding a racetrack, or any of numerous wading birds.
• Rail (verb): To revile or scold in harsh, insolent, or abusive language.
• Rail (noun, trains): The system of transportation that uses trains.
• Rail (verb, trains): To travel by railway.
• Rails (noun, engineering): A steel bar extending horizontally between supports which is used as a track for railroad cars or other wheeled vehicles.

In summary, rail can refer to a physical structure such as a bar or fence, a mode of transportation that uses trains, or a steel bar used as a track for railroad cars or other vehicles.