According to the national development strategy of the Greater Bay Area (GBA), the intercity railway network of the Pearl River Delta region will be further developed and will become the backbone of the GBA's transportation system in the next decade. However, similar to other transportation networks, human activities or natural hazards may cause disruptions to the railway network and lead to huge negative socio-economic impacts. Thus, to minimize the disruption risk for the railway network, the assessment of network vulnerability is indispensable. In the relevant studies, the research of vulnerability envelope (i.e., the upper and lower vulnerability bounds) can consider all possible combinations while avoiding the combinatorial complexity of enumerating multi-disruption scenarios, which is more suitable for assessing the railway network vulnerability. Specifically, the vulnerability envelope problem is formulated as a binary integer bi-level program (BLP). The upper-level subprogram maximizes or minimizes the remaining network travel throughput under a given number of disrupted links, which corresponds to the upper and lower vulnerability bounds. The lower-level subprogram checks the connectivity or usability of each origin-destination (O-D) pair under a network disruption scenario. By identifying the best and worst scenarios associated with geographical impact, this study assesses the latest regional railway network plan of GBA and presents some insights for the plan. As a new regional railway network plan is under planning in this year, the results of this study could provide decision-making support for the railway planners and managers.