Interesting way to run it, but it makes sense as the TPC is indeed putting the DC offset on to trigger the whistle/bell.
You might be able to still buy the command RS4 board from Lionel, they do have some boards listed on the website. I suspect most folks won't have a TPC in the mix if they're running command.
One of the reasons I started into the MTH stuff was that I could load multiple sound files to get different sounds and capabilities, with the TMCC conversions, you're limited to a very few generic sounds. This is a major failing of the Lionel upgrade market.
