Abstract: Apparatus for recording and displaying a graphic representation of the operating performance of a railroad locomotive by monitoring the response of its engine to a throttle control setting. The apparatus includes a chart paper recording device driven in relation to time and a marking mechanism actuated by a governor and referenced to its balance position at normal operation of the locomotive engine. The marking mechanism includes a pair of fixed styli and a movable stylus for producing a graph on the chart paper. The pair of fixed styli produce a pair of spaced-apart parallel datum traces on the chart paper wherein the area therebetween represents the normal governor balance position for a specified throttle setting. The movable stylus actuated by the governor produces a single trace on the chart paper relative to the parallel datum traces and represents the movement of the governor as it seeks to maintain its normal balance position.