In an assembly comprising a first railway rail side post insulator and a railway rail clip anchoring device (3), for retaining a resilient railway rail fastening clip (6), which device (3) is configured to support the first railway rail side post insulator (1'), the first railway rail side post insulator (1') is configured to provide an electrically-insulating load bearing face (10') between an upright side face (21 A) of a foot (21) of a railway rail (2) and upright load bearing faces (301, 302, 303, 304) of a railway rail clip anchoring device (3) located adjacent to the rail foot (21), the electrically-insulating load bearing face (10') being provided by an outer face of a wall (100') of the insulator. The insulator further comprises a member (110'), joined to an inner face (12') of the wall (100'), which member (110') has a top surface (11') which extends in a substantially perpendicular direction with respect to the load bearing face (10') to a depth (do) from the inner face of the wall (100') which is greater than or substantially equal to a depth (dw) of the wall in the said direction. The railway rail clip anchoring device (3) has upright load bearing faces (301, 302, 303, 304), comprising front wall faces (301, 303, 304) configured for location adjacent to the rail foot (21), and an upper central face (302) which is offset from the front wall faces (301, 303, 304) by an offset distance (D) which is between a tenth and a half of the depth (D0) of the anchoring device (3) in a direction substantially perpendicular to the front wall faces (301, 303, 304), and an upwardly-facing surface (31) of the anchoring device (3) which extends from the upper central face (302) to provide a shelf portion (31) for receiving part (43) of the first railway rail side post insulator (1').