In Japan, some monitoring systems have been introduced to evaluate conditions of the railway structures. In this work, we introduce the monitoring systems of railway bridges. Here, for example, we show the abstraction of fatigue damage monitoring system. The crack propagation properties of a side notched SUS304 thin sheet, which is affixed to a fatigue test specimen, is studied to develop a fatigue damage monitoring sensor. When cyclic loads are applied to the fatigue specimen, the side notched thin sheet behaves as a displacement controlled condition. The stress intensity factor (Delta) K is expressed by the following equation; (Delta) K equals (Delta) (sigma) (root)GL/2 where GL is the gauge length of the thin sheet. The thin sheet is fatigue pre-cracked, stress- relieved and affixed to a specimen. The results of fatigue crack propagation tests show that the crack propagation rate is dependent on stress range and gauge length, but not on crack length. Under sufficient mean stress conditions, the fatigue crack propagation rate is well expressed by a power low of (Delta) K even at such a low stress range as 20 MPa. Fatigue damage accumulated during a monitoring period can be estimated from crack propagation during the period. The sensitivity of the sensor is controlled through the gauge length of the sensor.