I am very pleased to report, that I can operate several features of an MTH steam engine with Proto Sounds 3.0 using a Lionel ZW-L Transformer!
I purchased an MTH UP 4-4-0 General #119. The bell, whistle, direction, and general operation work fine. The whistle is exceptional, and there are some "sounds" that are operable, including shut-down, although I haven't fully "auditioned" the sound repertoire yet.
At the outset, the set-up and even the simple operation of the locomotive was becoming a significant challenge. I could not get the "direction" controls activated for the locomotive, although I tried numerous possibilities, including fidgeting with the ZW-L's CMD/CNV switches.
I did, however do what I often do -- I re-read the operative sections of the MTH operator's manual. (I had already read the operator's manual earlier this morning.) The words, "temporary interruption of power" to activate the direction control gave me an idea. I decided to try to very quickly activate the direction button on my handheld -- or the direction slider on the ZW-L -- and that worked great for easily changing the direction. I hope that procedure continues to work!
