the vehicle car so the coupling force may be reduced to zero conversely if the sensed coupling force tends to decelerate the vehicle car the signal applied to the terminal 20 is at a level which tends to cause the braking system to apply the brakes on the vehicle car so the coupling force may be reduced to zero the input terminal 23 of the summing amplifier 19 receives a signal input from a demodulator 24 which is provided an alternating current signal input from a coupler transducer 25 which forms part of the coupler on the other end of the vehicle car this coupler transducer and demodulator system operates in a like manner to the coupling system formed by the demodulator 21 and the transducer 22 it follows therefore that the signal applied to the input terminal 23 must be inverted by the summing amplifier 19 since the coupling forces sensed by the respective transducers are opposite when providing the same polarity output that is when the transducer 22 senses a coupling force which tends to accelerate the vehicle the transducer 22 provides an output signal which is substantially the same as the output from the transducer 25 when latter transducer senses a coupling force which tends to decelerate the vehicle this will be explained in more detail later the summing amplifier 19 provides an output signal to an input 26 of the master control valve 3 this signal is a modified reference signal for controlling the movement of.