Most American company locomotives and rolling stock is compatible with each other to a certain extent. Since virtually everything is now made in china, I can't say American made, but American named importing companies. In the case of dcc the only thing that is interchangeable is the decoders. The NMRA wrote the standards for dcc so that all decoders would be compatible with all systems. If you install a dcc system on your home layout, you need to pick a system and stick with it. By making the decoders universal, it allows me to install a decoder in my locomotives to use on my home layout and then take those same locomotives to a club or a friends layout that has a different dcc system and still run my locomotives on his layout.
The main areas where rolling stock runs into compatibility issues is with copupler height, and it usually isn't very difficult to make the proper adjustments to get coupler height where it belongs.