OPRMS was correct, a string of cars parked anywhere, not only on a grade are secured with hand brakes. On railroad rolling equipment, all locos and cars have two brake systems. First is the train or independent brake that is controlled by an air compressor in the locomotive. That doesn't exist when a string is parked, so the crew sets hand brakes. Rule 7.2 of the GCOR mandates that a sufficient number of handbrakes will be set to prevent the string to roll away. If handbrakes aren't sufficient to keep the cars from moving, then use wheel blocks. Local special instructions will specify how many hand brakes will be set. When I set out cars no matter where, some hand brakes were set. There were also a few skates (wheel blocks) in the engine, so if I had to set out a string for storage on Altamont, then I would also put a skate under the lead car and roll the lead car up on the skate a little to prevent foamers and metal thieves from stealing them.