With the aim of performing perimeter surveillance of high-speed railway networks, this paper presents the design of a passive multistatic radar system based on the use of Long-Term Evolution (LTE) downlink signals as the illumination of opportunity. Taking into account the specifications and standard of the LTE system, the ambiguity function of measured downlink signals is analyzed in terms of range and Doppler resolution, ambiguities, and sidelobe level. The deployment of the proposed passive radar is flexible and scalable, and it is based on multichannel software defined radio receivers that obtain the reference and surveillance signals by means of digital beamforming. The signal processing and data fusion are based, respectively, on the delay-Doppler cross-correlation with the reconstructed reference signals and a two-stage tracking at sensor and central level. Finally, the performance of the proposed system is estimated in terms of its maximum detection range and simulation results of the detection of moving targets are presented, demonstrating its technical feasibility for the short-range detection of pedestrians, vehicles, and small drones.