Also, if it asks you to open/save file, what do you do and where do you put it?
In most cases the best policy is to save the file, virus scan it, then if it's ok run it/open it. That applies to anything you download, not just material for Trainz.
In more detail:
Either beforehand in Windows Explorer, or when the web browser program asks where to save a file, make a new folder somewhere, for example in "My Documents" (or elsewhere if you prefer), with a sensible name such as "TRS2004 Extras". Within that folder, create subfolders with a sensible naming scheme, for example for each type of asset, or for each web site.
After a download completes, go into Windows Explorer and view the folder where you saved the file, right-click on the file, and select your anti-virus program's option to scan it. No matter how much you trust the source, they might possibly be infected and not yet detected it (but this is highly unlikely with reputable web sites).
If the anti-virus program says it's ok, in most cases double-click it. If it's an exe file then it will run, and for Trainz assets probably install its contents. If it's a CDP file then it will cause Trainz' own installation program to run and install the contents. If it's a ZIP file you'll need to extract the contents using WinZip or a similar program and depending on what the contents are then see what to do next. For other file types you'll need to do some research to find out what to do, unless the web site you got it from explains.
Hope this helps and tells you something you didn't already know. Keep asking if you need more help,
John
