System for loading/unloading and rail transportation of road-hauled semi-trailers; system incorporating at least one railway wagon and a mounting for the kingpin of a semi-trailer, this wagon having a chassis and being intended to accommodate, for rail transportation, at least the road-hauled semi-trailer fitted with the kingpin at the front and a wheel assembly at the rear, with the wagon according to the system comprising: - at least one turntable that pivots in relation to the chassis, that is designed to accommodate the wheel assembly of the semi-trailer, this turntable able to be pivoted between a transportation position in which it is positioned longitudinally relative to the wagon, and a loading/unloading position in which it is oriented transversely or obliquely in relation to the wagon; and - at least one accommodating platform designed to accommodate the mounting for the kingpin of the semi-trailer and having an immobilizing system for immobilizing the said mounting; characterized in that: - the accommodating platform is connected to or integrated with the chassis, and is fixed in relation to this chassis thereto and independent of the pivoting turntable; - the mounting accommodated by the accommodating platform for the kingpin of the semi-trailer borne by the wagon is a detachable mounting; and in that the said detachable mounting comprises: - a body that is grippable by a handling machine and, for this, comprises a mechanical interface system that is compatible with the gripping system of the handling machine, a body which is assembled in a detachable manner on the accommodating platform of the wagon, and which comprises a complementary immobilizing system that interoperates with the immobilizing system of the accommodating platform to detachably secure and temporarily restrain the detachable mounting on the accommodating platform; and - a system designed to accommodate, support and immobilize the kingpin of the semi-trailer. - at least one turntable that pivots in relation to the chassis, that is designed to accommodate the wheel assembly of the semi-trailer, this turntable able to be pivoted between a transportation position in which it is positioned longitudinally relative to the wagon, and a loading/unloading position in which it is oriented transversely or obliquely in relation to the wagon; and - at least one accommodating platform designed to accommodate the mounting for the kingpin of the semi-trailer and having an immobilizing system for immobilizing the said mounting; - the accommodating platform is connected to or integrated with the chassis, and is fixed in relation to this chassis thereto and independent of the pivoting turntable; - the mounting accommodated by the accommodating platform for the kingpin of the semi-trailer borne by the wagon is a detachable mounting; - a body that is grippable by a handling machine and, for this, comprises a mechanical interface system that is compatible with the gripping system of the handling machine, a body which is assembled in a detachable manner on the accommodating platform of the wagon, and which comprises a complementary immobilizing system that interoperates with the immobilizing system of the accommodating platform to detachably secure and temporarily restrain the detachable mounting on the accommodating platform; and - a system designed to accommodate, support and immobilize the kingpin of the semi-trailer.