What is the difference between the two?
The K and AB valves perform the same functions. AB valves have some additional steps in the operation to allow for more uniform performance, and less sensitivity to minor brake pipe pressure changes (which helps prevent "dynamiters", or valves that kick into emergency braking unintentionally), along with quicker release and recharge.
K brakes haven't been permitted in interchange service for decades, but will interoperate with AB if they're in the same train.
Check out this thread from years ago. Paul North provided some links to information about brake types:
http://cs.trains.com/trn/f/111/t/186509.aspx
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