the identifier component 720 may also include a component uniquely representing the locomotive which is being controlled the integrity component 730 which is optional is used to assist the controller 46 in determining whether it has correctly received the function component 710 a 710 b and the identifier component 720 of the locomotive control word 700 in one embodiment the integrity component 730 may take the form of a cyclic redundancy check crc referring back to fig3 in order to assemble the various components of the locomotive control word 700 an embodiment of the present invention provides the command signal generator 38 with a microprocessor that runs a program stored on a computer readable medium in a different form of construction the command signal generator 38 may be constituted by an array of hardwired logic gates that generate the locomotive control word upon actuation of the controls operation of the microprocessor in accordance with its program is now described in greater detail with reference to the flowchart in fig4 at step 410 the microprocessor records the state of conduction of the electric contacts of the transmitter controls this may be done on a periodic basis or upon a change being detected in one of the actuators at step 420 the microprocessor checks the operational status of the portable communications unit 10 on the basis of the identity of the closed contacts and on the basis of the operational status of the portable communications unit 10 the microprocessor will produce either the full length function component 710 a or the shortened function component 710 b of the locomotive control word 700 specifically if at step 420 the microprocessor determines that the operational status.