The Indianapolis Colts on Sunday will play a playoff game at Baltimore -- their former home. What's the only other franchise that's done this (played a playoff game in a city where it used to be located)? This is a difficult question. The playoff game in question was played in 1950.
The St. Louis Rams (previously the Los Angeles Rams and earlier the Cleveland Rams). As the LA Rams, they played the Browns in the NFL championship game in Cleveland in 1950.