In order to ensure the traffic safety of high speed train, it is necessary to deploy wireless sensor networks in viaduct, cutting and tunnel space along the railway for comprehensive environmental monitoring. However, there are few studies on low energy routing protocol for wireless sensor networks in three-dimensional space. This paper presents a 3DAK (3D advanced K-means) routing protocol based on three dimensional advanced K-means clustering algorithm, which calculates the minimum node density by constructing Reuleaux tetrahedron model to ensure the communication connectivity between nodes. We can obtain the optimal wireless sensor node density and cluster average size through simulation results. Moreover, the cluster heads are determined by combination of the residual energy and the location of nodes. Simulation results show that 3DAK routing protocol can better save energy, prolong the network lifetime and improve the survival number of nodes compared to other classical routing protocol.