In this paper, we extend the concept of the Operational Design Domain (ODD) to the context of railway remote driving in order to guide the safety demonstration process. Mainly, we propose an iterative process to define and to specify an ODD for the railway remote driving based on the operational risk assessment process. The main idea consists in, starting from an initial high-level ODD, including the overall operational environment of conventional in cab driving, iteratively restrict and refine the ODD by exploiting the risk assessment steps (particularly, the risk evaluation and the risk reduction) through feedback loops to the ODD.