Due to land scarcity in greater Taipei area, continuously conversion from existing railroad to underground are performed to increase the land utility at different phases and span many years. In addition, integration issues between several transportation systems such as mass transit and high speed rail are also of great concerns. Fourth phase of Nangkang North Tunnel was completed and in full operation in September, 2008. During the construction period, fire codes in Taiwan for special constructions (e.g. ubnderground stations) are also going through various revisions while internationally, new regulations such as AS 4391 in Australia and NFPA130 in USA have been validated and updated. In order to verify the compliance of fire codes nationally and internationally, full scale experiments have been performed before opening to public service with emphasis on various operation modes of mechanical ventilation. This paper aims to numerically investigate and compare the effectiveness of CFD (Computational Fluid Dynamics) with experimental results. As a matter of fact, full scale experiments are implemented in fire codes in Taiwan for the structures which cannot fully comply with normal installation of fire equipments. While computational power and resources are improving tremendously in the past few years, we focus on the potentials to gain insightful information through numerical simulation and provide a repeatable reference for the engineers, designers and fire fighters as well as managerial levels in government before undertaking any full-scale experiment.