Results of a diagnostic load test and in-service monitoring of a steel through-girder bridge are presented. The bridge currently has a low load rating that is controlled by the flexural capacity of the steel deck trough. The diagnostic load test was conducted using the regularly scheduled transit trains, with no interruption to the daily service. A total of 16 strain transducers were mounted to the bridge girders and trough; five dynamic train passes were recorded. The largest tensile and compressive stresses recorded in the edge girder were 33.5 MPa and 23.4 MPa, respectively; the largest tensile and compressive stresses in the deck trough were 25.9 MPa and 27.9 MPa, respectively. The test results showed the effective width of the deck trough to be 2.89 m for a single axle and 5.19 m for a 2.74-m spaced axle pair. An in-service monitoring system was also placed on the bridge to record the stress cycles experienced by the bridge over a 1-week period. During the in-service monitoring period, 163 trains crossed the bridge with an associated 1,456 axles. The mean peak tensile stress in the deck trough caused by locomotives crossing the bridge was 26 MPa with a standard deviation of 1.8 MPa. The test results showed that the peak deck trough strains due to live load were only 15% of the computed values used in the rating. As a result, the bridge load rating can safely be increased, and the current speed restrictions can be lifted.