Looking for suggestions as to what or type of file I should include there. I have just included a Railway Texture Mega Pac for example.
Buck
Do you mean which plugins you should place in the install dir Plugins folder? As opposed to the Plugins folder in Documents?
The additional folder in the install directory was not really intended for additional content. Ideally all such files should be placed into the user Plugins folder, i.e. the one in Documents\SimCity 4\Plugins. The only exceptions are .dll files, which may not correctly initialise unless loaded before all other content, so it's advisable to place them into Plugins in the install directory instead. Whilst mods placed into the Install dir Plugins folder will be seen by the game and work, it's really best avoided to do this unless you really understand what you are doing.
On a similar note, the Regions folder in Documents is the only one you can use. The second in the install dir is only there to keep a backup of the original regions that shipped with the game. If you place custom regions here, the game won't load them, if you mess with the files there, you may need to re-install the game to fix it.
Thank you