I spent five years of my life working as a Railroad Conductor in freight service. It was a short line railroad that runs between the Canadian border at St. Albans, VT and New London, CT. One night around 11:30PM I was spotting a car on a customer's track in Brattleboro, VT, near the Connecticut River. I had my engineer cut away from the train, on the main line holding onto 15 cars. We backed onto the customer's track after I removed a steel derailing device. It is on their side of a crossing; route 142. The customer's unloading facility is on a hill. If a car got loose, it would go over the derailing device and stop on dirt rather than crossing the road and strike an automobile. That would be bad. After tying the boxcar onto a second railcar already on the track, I checked the handbrake on the existing car and felt that was enough to hold both cars in place. I then pulled away from the two empty boxcars. The air dumped on the boxcars, which causes the brakes to set up. I had the engineer stop just beyond the derail. Then I stepped between the rail to put the derail back on. I heard a noise and looked to my right. Here came the car I just set off rolling at me at about 3 or 4 MPH. That might not appear fast, but a steel object weighing 25 tons, it could do serious damage including knuckling me between two cars. That would be fatal. I dove out of the way while yelling an expletive and the car ended up going on the ground after rolling over the derail I replaced. Apparently, there was a problem with the air reservoir on the rolling car. The air dumped in both reservoirs leveling the brakes and making them not work properly. The pin which would hold the cars together, locking the knuckles in place did not drop. Because I did not put a hand brake on the car, it rolled free. It was scary and I did not get fired as a result.