In many rail networks, infrastructure constraints force the shared usage of lines between passenger and freight movements. Scheduling additional freight movements around existing passenger services and peak traffic based curfews presents significant challenges to commodity industries eager to increase export volumes. This paper addresses the problem of inserting additional freight movements in a constrained railway network. To this end, a railway operations planning model was developed to simulate and insert feasible rail movements in a non-periodic timetable. The simulation modelling platform developed in this paper is called RailNet, which simulates the existing railway network constraints and is capable of adding freight paths for planning and scheduling. The timetable for passenger trains is kept unchanged. The paper also reports a real case study in which RailNet was used to quantify the capacity of the track network at the Port Kembla Coal Terminal in New South Wales, Australia under different scenarios of infrastructure upgrades.