A variable diameter fastener for a railway rail fastening assembly comprises a shaft, a first end of the shaft being configured for placement in an underlying foundation and a second end of the shaftbeing configured for engagement with the fastening assembly. The shaft comprises a variable diameter portion between the first and second ends. The variable diameter portion has a diameter that variesalong a longitudinal axis of the shaft such that an outer surface of the variable diameter portion is convex in a plane containing the longitudinal axis of the shaft.