Natural gas transport networks have the primary function of transporting natural gas over long distances. Due to the compressibility of gas, a sufficient line pack must exist in the network to provide the contractual delivery pressures. Based on the fact that pressure levels are mostly specified as pressure bands, there is also the possibility to store gas within the network. The gas volume which can be stored and is not necessary for the transport is the line pack flexibility. In context of the European liberalized market the network operator has the function of managing the gas transport of different actors. Because of forecast deviations, imbalances between gas feed-in and gas feed-out often occur. This is compensated by the use of balancing energy, which can be provided either from external sources or from the line-pack flexibility. As the necessary line pack for transport varies with each transport situation, this volume must be balanced by the line-pack flexibility. Therefore, a precise knowledge of the line-pack flexibility during the network operation is very important. For this purpose computer-aided simulations are necessary. Because the increase or decrease of the line-pack flexibility is a transient process, transient simulations should be used. However, transient simulations require a long computing time, which leads to many disadvantages during the network operation. Therefore, this paper develops a method to simulate a natural gas transport network by using fast steady-state calculations under consideration of the transient characteristics of the line-pack flexibility. For this purpose the line-pack flexibility and transient calculation models are analyzed in detail to show the problems using a steady-state approach. Afterwards a method is described to take into account the transient characteristics of the line-pack flexibility. This is verified by exemplary simulations. Finally, the main findings are summarized.