FAAJAA,
The ATP like the Flight Engineer writtens may or may not expire after 24 months. According to FAA Washington Legal, if the holder of an ATP or FE written works for a Part 121 or Part 135 commuter, while the written is within the 24 month period and is working for a Part 121 or Part 135 Commuter and has gone through the carriers training program outside of the 24 month period, the written is still valid. Does that clear up the issue? Like most of us, probably not.
Richard