I ran a test in my TRS22 (build 128486) of both the decoupler and the retarder and everything appears to be working fine. My boxcar was decoupled and propelled to the speed I set and the retarder slowed the boxcar down to the speed I set. My locomotive speed was 3MPH when it approached the decoupler.
Questions: Did you set the trigger distance for each component to what I recommended (ie; 1.5 meters) in the tutorial?
Bob