In 2020, Indian Railways took up a de novo timetabling exercise for the Golden Quadrilateral and Diagonals (GQD) connecting Mumbai-Delhi-Kolkata-Chennai, involving about 1700 trains on a weekly basis over a network of 9,099 km. Based on significant participation in this activity, this paper elaborates on the challenges faced in a network-wide simulation and scheduling of trains across a large network. The methods to address these challenges and the software implementation aspects are also described. A network-wide mixed-rail traffic simulator was used successfully to construct a daily timetable for the GQD. The two primary goals of the Zero Based Timetabling (ZBTT) project were to generate train schedules that were: (a) feasible (in terms of kinematic constraints on both trains running and block-section, running lines) and (b) conflict-free (due to multiple trains having to share resources and the need to ensure time-durations of resource usage are different, subject to safety constraints), for a proposed set of departure timings and halt patterns of all the services that were planned on the GQD.