The SATLOC project aims at demonstrating an innovative concept of applying the GNSS to railway safety and advanced operation on low traffic lines. One of the issues is to determine if GPS, Galileo and EGNOS are available along the track and can answer to the railway requirements even in the presence of masking obstacles along the track. This paper presents the simulation approach developed by Spirent and Ifsttar in order to simulate GNSS signals in a realistic masking environment known by image acquisition and processing. The paper illustrates the concept with results obtained at a tunnel output near to Brasov, Romania, and concludes with perspectives that could enrich the results, in particular by including multipath effects and not only line-of-sight satellites.