Every cable TV network needs one thing to thrust itself into the consciousness of viewers: a signature show that attracts ratings and passionate advocates.

During the past three months, WGN, a channel that has lacked an identity for much of its more than three decades, has scored an unusual success with its first original drama, “Salem,” generating strong reviews and positive word of mouth.

Too bad it is not a cable network.

Not yet, anyway. WGN America, as it is now called, is among the few remaining “cable superstations,” holdovers from cable’s early days when a few large local broadcast stations, relying mostly on sports — like Chicago Cubs games on WGN — carved out niches on cable systems, though they were not categorized as cable networks.

“Salem” is the first step in a strategy by the Tribune Company, owner of WGN America, to transform the superstation into a full-fledged cable network, creating yet another competitor in the increasingly crowded field of purveyors of distinctive television drama. Next up, in July, is “Manhattan,” a drama about the secret work to develop the atom bomb.