I got this straight fromKristen at MTH when I acquired about the transition point from 5 volt PS2 to 3 volt PS2.
"Thank you for contacting MTH Electric Trains. We appreciate your interest in our products. I am afraid I cannot give you an exact date or catalog where we switched from the 5-volt ProtoSound 2.0 to the 3-volt system. For the Premier engines (steam and diesel) in was primarily in the 2004 volume 1 catalog; the majority of the engines offered in that catalog or after have the 3-volt system. For the RailKing engines the switch over was about one catalog later - the 2004 volume 2 catalog. If you have engines that were released around this time we can check on a specific models.
Please let me know if you have any questions."
Hope it helps some.