The present invention provides a positioning and pressing device, a pressing device and a tool for auxiliary machining of motor train underframe, the positioning and pressing device comprises a crossbeam, two supporting frames, two rotary vertical pressing structures, a lateral positioning structure, a lateral holding structure and two adjustable intermediate supporting structures; the two supporting frames are mounted in sliding manner on two sides of the upper surface of the crossbeam; the two rotary vertical pressing structures are respectively mounted on the tops of the two supporting frames; the lateral positioning structure is mounted on the side of one of the two supporting frames; the lateral holding structure is mounted on the side of the other one of the two supporting frames; and the two adjustable intermediate supporting structures are mounted on the upper surface of the crossbeam and between the two supporting frames. The present invention realizes that the tool for the auxiliary machining of the motor train underframe is modularized; the present invention solves the technical problems of the present technology that the motor train underframe machining structure is large in volume and is not convenient for transportation and movement and the adjustability thereof is not good and not capable of meeting the requirements of the auxiliary machining of the motor train underframe with different types.