EMPLOYEE MOVED RAMCAR 688 NEAR DIESEL STATION TO REFUEL; WHILE WAITING HIS TURN, HE REMOVED TRASH FROM RAMCAR CAB; WHILE ATTEMPTING TO DISMOUNT THE RAMCAR, HIS RIGHT FOOT SLIPPED OFF THE LOWER STEP AND HE FELL BACKWARDS OFF MACHINE. HE TRIED TO CATCH HIMSELF BY REACHING OUT WITH HIS RIGHT HAND, CAUSING HIM TO FALL WITH HIE WEIGHT ON HIS RIGHT ARM--CAUSE OF RIGHT ARM PAIN.