EMPLOYEE WAS OPERATING AN EIMCO 935 LHD SCOOPTRAM. HE SET THE PARK BRAKE AND WALKED BEHIND THE SCOOPTRAM TO HELP PIN THE CONVEYER CHAIN OF AN EIMCO CONT MINER. THE PARK BRAKE FAILED ALLOWING THE SCOOPTRAM TO ROLL, PINNING THE OPERATOR BETWEEN THE REAR OF THE SCOOP TRAM AND THE END OF THE BOOM OF THE CONTINUOUS MINER.