A rail system for use when assembling a component in an aircraft, which comprises: - a first rail element having a securing device which is designed to be complementary to a first securing device provided in the region of a floor of an aircraft fuselage element and is adapted to cooperate with the first securing device in order to releasably secure the first rail element in the region of the floor of the aircraft fuselage element, wherein a section of the first rail element projects beyond an edge of the floor of the aircraft fuselage element and is adapted to extend over a floor of an assembly hangar in the connected state of the first rail element and the aircraft fuselage element, and having a guide device, which is designed to be complementary to a first guide device provided on an assembly aid, which is to be introduced into the aircraft fuselage element, and is adapted to cooperate with the first guide device to enable a guided displacement of the assembly aid to be introduced into the aircraft fuselage element relative to the rail system, and - a second rail element having a securing device which is designed to be complementary to a second securing device provided in the region of the floor of the aircraft fuselage element and is adapted to cooperate with the second securing device in order to releasably secure the second rail element in the region of the floor of the aircraft fuselage element, wherein a section of the second rail element projects beyond an edge of the floor of the aircraft fuselage element and is adapted to extend over a floor of an assembly hangar in the connected state of the second rail element and the aircraft fuselage element, and having a guide device, which is designed to be complementary to a second guide device provided on an assembly aid, which is to be introduced into the aircraft fuselage element, and is adapted to cooperate with the second guide device to enable a guided displacement of the assembly aid to be introduced into the aircraft fuselage element relative to the rail system. - a first rail element having a securing device which is designed to be complementary to a first securing device provided in the region of a floor of an aircraft fuselage element and is adapted to cooperate with the first securing device in order to releasably secure the first rail element in the region of the floor of the aircraft fuselage element, wherein a section of the first rail element projects beyond an edge of the floor of the aircraft fuselage element and is adapted to extend over a floor of an assembly hangar in the connected state of the first rail element and the aircraft fuselage element, and having a guide device, which is designed to be complementary to a first guide device provided on an assembly aid, which is to be introduced into the aircraft fuselage element, and is adapted to cooperate with the first guide device to enable a guided displacement of the assembly aid to be introduced into the aircraft fuselage element relative to the rail system, and - a second rail element having a securing device which is designed to be complementary to a second securing device provided in the region of the floor of the aircraft fuselage element and is adapted to cooperate with the second securing device in order to releasably secure the second rail element in the region of the floor of the aircraft fuselage element, wherein a section of the second rail element projects beyond an edge of the floor of the aircraft fuselage element and is adapted to extend over a floor of an assembly hangar in the connected state of the second rail element and the aircraft fuselage element, and having a guide device, which is designed to be complementary to a second guide device provided on an assembly aid, which is to be introduced into the aircraft fuselage element, and is adapted to cooperate with the second guide device to enable a guided displacement of the assembly aid to be introduced into the aircraft fuselage element relative to the rail system.