System for locking an access door to a space, especially the toilets of a railway vehicle, the closed door extending in a plane defining an inner side of the system oriented towards said space and an outer side oriented away from the space, the system comprising: - a framework which is configured to be fixed in a frame of the door or in the door and which defines at least in part an inner aperture situated on the inner side and an outer aperture situated on the outer side; - an inner locking plate having an outside face facing towards the inner side and an inside face facing towards the outer side, the outside face having a first visual, and advantageously tactile, sign representing a "free" state of said space and a second visual, and advantageously tactile, sign representing an "occupied" state, the inner locking plate being mounted to be rotatable about an axis relative to the framework between an open position, in which the first sign is aligned with the inner aperture, and a closed position, in which the second sign is aligned with the inner aperture, the axis being substantially perpendicular to the plane; - a locking knob which is situated on the inner side and configured to drive in rotation the inner locking plate; - an outer locking plate which is substantially parallel to the inner locking plate and has an outside face facing towards the outer side and an inside face facing towards the inner locking plate, the outside face having a third visual, and advantageously tactile, sign representing the "free" state and a fourth visual, and advantageously tactile, sign representing the "occupied" state, the outer locking plate being secured in rotation about the axis with the inner locking plate between an open position, in which the third sign is aligned with the outer aperture, and a closed position, in which the fourth sign is aligned with the outer aperture; - a dead bolt, and a linking system which mechanically connects the dead bolt to the inner locking plate or to the outer locking plate; - an outer manual disabling plate having an inside face substantially parallel to the outside face of the outer locking plate, and an outside face which faces towards the outer side and has a fifth visual, and advantageously tactile, sign representing a "manual out of service" state of the space, the outer manual disabling plate further defining a first window; - a lock which is situated on the outer side and configured to drive the outer manual disabling plate in rotation; characterised in that the inside face of the outer manual disabling plate is situated opposite the outside face of the outer locking plate, the outer manual disabling plate being rotatable about the axis relative to the framework between an out of service position, in which the fifth sign is visible through the outer aperture, and a neutral position, in which the first window is aligned with the outer aperture, the outer manual disabling plate having a stop which is configured to put the outer locking plate in its closed position when the outer manual disabling plate is in the out of service position. - a framework which is configured to be fixed in a frame of the door or in the door and which defines at least in part an inner aperture situated on the inner side and an outer aperture situated on the outer side; - an inner locking plate having an outside face facing towards the inner side and an inside face facing towards the outer side, the outside face having a first visual, and advantageously tactile, sign representing a "free" state of said space and a second visual, and advantageously tactile, sign representing an "occupied" state, the inner locking plate being mounted to be rotatable about an axis relative to the framework between an open position, in which the first sign is aligned with the inner aperture, and a closed position, in which the second sign is aligned with the inner aperture, the axis being substantially perpendicular to the plane; - a locking knob which is situated on the inner side and configured to drive in rotation the inner locking plate; - an outer locking plate which is substantially parallel to the inner locking plate and has an outside face facing towards the outer side and an inside face facing towards the inner locking plate, the outside face having a third visual, and advantageously tactile, sign representing the "free" state and a fourth visual, and advantageously tactile, sign representing the "occupied" state, the outer locking plate being secured in rotation about the axis with the inner locking plate between an open position, in which the third sign is aligned with the outer aperture, and a closed position, in which the fourth sign is aligned with the outer aperture; - a dead bolt, and a linking system which mechanically connects the dead bolt to the inner locking plate or to the outer locking plate; - an outer manual disabling plate having an inside face substantially parallel to the outside face of the outer locking plate, and an outside face which faces towards the outer side and has a fifth visual, and advantageously tactile, sign representing a "manual out of service" state of the space, the outer manual disabling plate further defining a first window; - a lock which is situated on the outer side and configured to drive the outer manual disabling plate in rotation;