Are you talking about developing for trains? There are a handful of train plugins out there that use DLL's, but they are no longer encouraged, because of backwards compatibility.
Take a look at our wiki for an API reference under Plugins.
http://wiki.bvestation.com/index.php/Main_Page Unfortunately, not many of us have experience with it, but if you do run into issues, we'll try to help as much as we can.
The v1.4.3 source code appears to be using the new plugin system, but I myself haven't explored that section of the code.