The second person is there to protect the train by going back 1.25 miles and laying the detonator caps on the rail if anything untoward happens. Typically they find a lineside phone and call in from there and the signalman protects the train. Basically someone to secure the train and someone to get help
With a secure radio system with full coverage any train can contact the signalman directly, signalman can contact any train he wants directly using the trains ID code as the phone number. The IE system even allows text messages to be sent and can allow direct dial so you can ring a landline from a the cab (fabulously clever for early 1980's).
In Northern Ireland the guard is the conductor as well and closes the doors (even though the driver can). Safety rules requires that if any of the cab safety equipment fails a second 'qualified' person travels with the driver that applies pretty universally the world over
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