The K-Line with cruise uses the same three Lionel boards, the R2LC, the RailSounds audio, and the RailSounds power supply. However, instead of the DCDR motor driver, they have the K-Line cruise board. That's usually the troublesome board. Here's a clip from a thread here, the K-Line cruise board is at the top, and the ERR Cruise Commander M is at the bottom. The CC-M is what we typically use to replace the K-Line cruise.
