I was going to go into the history a bit but I'll take this opportunity to shill for Fybush and jsut quote the master directly:
"At Saxonburg, KDKA experimented with shortwave broadcasting, with medium-wave power levels as high as 400 kW and with several antenna designs that were complex (and apparently unsuccessful) prototypes of today's low-profile antenna designs, such as the Kinstar and Paran antennas. The Saxonburg site remained in use for shortwave (under the W8XK calls) until the end of the war, and was eventually donated to the nuclear physics program at [Carnegie Mellon.]"8XK was Dr. Frank Conrad's old experimental call from 1916 it was canceled in 1917 and re-licensed as 8XK in 1920. He began broadcasting KDKA programs on shortwave in 1923 as 8XS. Westinghouse was attached to the calls so when they launched a WBZ in Boston, it's first experimental calls were W1XK, and WBOS which became W2XK. In the 1930s all shortwave was still experimental and all of them started with experimental calls in this era. All experimental calls consisted of the letter "W" followed by a number representing the "radio district" then an "X" then two or three letters. More here.
Westinghouse had to hand over the reins to The Government in WWII for operation under the Voice of America network. In 1939 the experimental W1XK calls were replaced with the more traditional calls WPIT.