I am trying to get the same code to compile/work under Windows and Linux (and probably OSX) and I'm wondering what this DLL does and why (if needed) it can't be rewritten into C#.

Any ideas?

Probably, it could be rewritten into C#, but my attempts to do so failed at the time I wrote the code. The problem is associated to calling conventions, over which I have more control with C/C++. However, there is no point in considering to rewrite the code into C# as the proxy has the sole purpose of communicating with the Windows-only train plugins (DLL files), whose machine code cannot be run natively on other platforms anyway. (You can of course get to run those plugins with Wine on Linux, but then, you don't need to rewrite the proxy either.)

Right, also, I can't get the Multi platform folder to build at all (missing resources), however by defining LINUX and removing the windows definition allows me to build the "Win32" folder which leads me to belive that having duplicate sources is unneccesary and it'd be much better to have a build system that can work on both (perhaps the autogen for linux?), the only problem I have now is that Network West Midlands seems to take ages load on linux, need to investigate this some more...

The source code is the same in the two projects. I am only providing two folders because SharpDevelop (and I suspect MonoDevelop too) does not like the binary XML resource files generated by Visual Studio and refuses to open the Visual Studio project as a consequence.

By the way, I can perfectly compile for all three platforms using SharpDevelop. I suspect though that you are using MonoDevelop, which is not available on Windows, so I can't test with that program.

The IDE is irrelevant, I compiled using gmcs on Linux, Mono wouldn't build the Mono release, only the Win32, which suggests to me that the Mono folder is unnecessary.