The library in the back

There is a bookcase on the back wall of the carriage. Next to it, on the floor, there are some marks. They have a semicircular shape, like the ones left by a door that grazes the floor when it opens or closes. You try to move the library, but you cannot. It looks anchored to the wall. Is it a secret door? You start emptying it. Hidden behind some books, there is a keyboard and a screen requesting a three letter code. Will you be able to open the door by entering the correct code?