My late father, a WWII veteran, saw a UFO along with many other people while a worker on a local railroad in 1951.

My father ( 1926-97) recounted the following sighting to me in the 1960s, 70s and 80s:  
He recalled that he was a day laborer on the Conemaugh-Blacklick Railroad working with a sledgehammer on the track.  He had been on the job for several days on a crew of dozens to perhaps a hundred or more workers.  It was a sunny, cloudless day with a deep blue sky.  He guessed that it was mid-morning to mid-afternoon.  
All of a sudden, someone said something like:  "Oh look!"  He looked up and saw a gigantic silver disk travelling at an incredible speed.  People gasped in awe.  After reaching a point in the sky, it came an immediate halt, hovered for many seconds to maybe a minute, then dropped a mile or two, hovered, shot across the sky, hovered, then disappeared.  It made no sound and left no vapor trail.  It had no ornamentation, but was a clean disk.  
Dad had never seen a UFO before and did not believe in them, nor was he a particularly religious or superstitious person.  He had been discharged from the Navy in December 1949, where he had driven a landing cra! ft since 1944.  He said he had been trained to judge size and distance and had used antiaircaft guns in some of his engagements in the South Pacific, as well as his ship's 14-16" guns as a fill-in.  He estimated this UFO was approx. the size of four football fields, and travelled at speeds exceeding any aircraft he had ever see, which included some of the early jets.  Only a missile well into its flight might have equalled it.  But it stopped on a dime!  
The next day, he came to work and was told he was no longer needed.  He had not  checked to see if it was reported by the local media and had not gotten acquainted with anyone, but he did recall talking about it with somone who worked next to him.  Later, he did see the movie, "The day the Earth Stood Still," and remarked that it looked alot like that craft, but he said it was much larger.
Source of report indicates that the date is approximate.  PD