A method for replacing a rail for guiding a piece of equipment inside a nuclear reactor: - the nuclear reactor having a pressure vessel with a central axis, in which the piece of inside equipment (10, 12) is placed, the original rail cooperating with a guideway to guide the piece of inside equipment (10, 12) relative to a reference element (2, 13) being the pressure vessel or being situated in the pressure vessel; - the original rail being fastened to one of the inner piece of equipment (10, 12) and the reference element (2, 10), the guideway being fastened to the other of the inner piece of equipment (10, 12) and the reference element (2, 10); - the original rail having substantially radial side faces with surfaces altered by rubbing against the guideway, and sound surfaces (60, 62, 64) not altered by rubbing against the guideway; the method comprising the steps below: - creating a fixed reference; - determining an original circumferential position of the sound surfaces (60, 62, 64) of the original rail around the central axis relative to the fixed reference; - disassembling the original rail; - circumferentially positioning a replacement rail around the central axis relative to the piece of inside equipment (10, 12) or the reference element (2, 10), using the original circumferential position of the sound surfaces (60, 62, 64) obtained during the determination step; - fastening the replacement rail to the piece of inside equipment (10, 12) or the reference element (2, 10). - the nuclear reactor having a pressure vessel with a central axis, in which the piece of inside equipment (10, 12) is placed, the original rail cooperating with a guideway to guide the piece of inside equipment (10, 12) relative to a reference element (2, 13) being the pressure vessel or being situated in the pressure vessel; - the original rail being fastened to one of the inner piece of equipment (10, 12) and the reference element (2, 10), the guideway being fastened to the other of the inner piece of equipment (10, 12) and the reference element (2, 10); - the original rail having substantially radial side faces with surfaces altered by rubbing against the guideway, and sound surfaces (60, 62, 64) not altered by rubbing against the guideway; the method comprising the steps below: - creating a fixed reference; - determining an original circumferential position of the sound surfaces (60, 62, 64) of the original rail around the central axis relative to the fixed reference; - disassembling the original rail; - circumferentially positioning a replacement rail around the central axis relative to the piece of inside equipment (10, 12) or the reference element (2, 10), using the original circumferential position of the sound surfaces (60, 62, 64) obtained during the determination step; - fastening the replacement rail to the piece of inside equipment (10, 12) or the reference element (2, 10). - creating a fixed reference; - determining an original circumferential position of the sound surfaces (60, 62, 64) of the original rail around the central axis relative to the fixed reference; - disassembling the original rail; - circumferentially positioning a replacement rail around the central axis relative to the piece of inside equipment (10, 12) or the reference element (2, 10), using the original circumferential position of the sound surfaces (60, 62, 64) obtained during the determination step; - fastening the replacement rail to the piece of inside equipment (10, 12) or the reference element (2, 10).