This happen when I was working on the old PRR. I was called to protect the job of head brakeman on a intercity industrial switch job.I got sign in with the other members of the crew and was told by the conductor to go with Fred and Charlie(engineer and fireman-yes diesels at that time still had fireman).So,we walk over to the engine out bound ready track to pick up the unit assign for todays train.We was told the unit will be 96XX A F7A- not the best unit for a local..A start of a very bad day as it would turn out..So we picked up the unit and proceeded to our short train-9 cars coupled up and begin pumping up the air.It would be 45 minutes before we got clearance to proceed out of the yard and head for the industrial branch..Rain began to fall for the third day in a row.. We made it as far as Scioto Tower and was greeted by a red block..We had to wait on a C&O man to cross the diamonds.We finally got the green to proceed after the C&O man cleared the diamonds.We proceed up the branch and did our assigned work at this point all is going fairly well considering the F7. So after we stopped at a Mom & Pop grocery store located by the tracks to get some sandwiches and pop for lunch.We ate lunch and continue the days work.Trouble was brewing. We finish our work and it was decided not to run around our train because of the F7 would be facing the wrong way we would simply back to the Cleveland Ave yard with the rear brakeman protecting our reverse move from the platform of the cabin car.This we have done many times..Fate was about to play her hand. We proceeded to return to the junction where we would enter the main when I looked back over the train from the cab and shouted FRED!!!! Emergency!!! You see I saw a 40' boxcar leaning over-to late.Before Fred could stop the train 4 of the 7 cars left the rails.. After the normal investigation it was found that the rail had simply roll over from years of little maintenance,no drainage,earth covered rotten ties and no ballast..You see the rain had soften the ground and the weight of the cars shove the rail over some on the way up the branch and finished the job on the way back causing the derailment..Of course it never came to light that the rear brakeman was protecting the reverse move from inside of the cabin and by looking out the door window.He would step out and sound the caboose whistle when we neared a street crossing or flag the crossing if he had to according to the time table rule book..Needless to say there was 3 happy faces in the cab of that old F7.You see had we had a EMD switcher(the normal type unit assigned to this run) or a road switcher such as a SD7 or 9 the unit would have left the rails instead of the cars.