Q, X and Z IATA codes are typically bus or rail stations to enable through tickets to be issued pursuant to air/rail or air/bus agreements.

In the past, QKB was the IATA code for United's former Landline bus service from DEN to Breckenridge. As you may recall, ZFV was 30th Street Station in Philadelphia, a little "hack" that many used to good effect during the days of the now-expired Amtrak partnership.