Railway control systems are based on on-board and trackside subsystems for signaling purposes. Several factors demand for new design and implementation solutions for such railway control systems. These factors are related to the design of interoperable railway networks in Europe, the introduction of new technologies and equipment, and the competition in the market of railway products. The safety of such new design and implementation solutions should still be proved in accordance with the CENELEC recommendations. In this paper SFDA, Safe message Freshness Detection Algorithm among trackside subsystems, is deeply described. SFDA is included in a new message passing safety protocol stack and it allows the detection of "old" messages and the meeting of real time and safety requirements of trackside railway systems. It is demonstrated that the SFDA can detect all the old messages. Moreover a preliminary analysis of its availability characteristics to check whether it is suitable for railways systems is performed through simulation.