A rail stress monitoring system is disclosed. This system includes a sensor module that further includes a sensing device that is adapted to be mountable directly on a length of rail. The sensing device further includes a generally flat metal shim and at least one, and typically two or more, sensors mounted on one side of the shim. The sensors are typically strain gauges, which are mounted on the shim in a specific, predetermined configuration. At least one data acquisition module is in electrical communication with the sensing device and a data processing module receives and processes information gathered by data acquisition module.