A pressure differential-driven engine comprises an outer pressurizable enclosure and an actuator enclosure, disposed within the outer enclosure. An actuator is disposed within the actuator enclosure and a portion of the actuator and the actuator enclosure cooperatively define a pressurizable cavity cyclable between a first, high pressure state, and a second, low pressure state. The actuator and actuator enclosure are collectively restrained by at least one rail to linear, slidable motion within the outer enclosure. The engine is operable to output usable energy as the pressurizable cavity cycles between the first and second pressure states.