You're right Dave, Trainz Objectz lists -10:199 as being a built-in item in TRS2004 SP4: "54ft_covered_hopper [PS]".
So the following should fix it, hopefully: go to http://www.auran.com/TRS2004/ then click "Downloads" (not "Download Station", that's a different facility) near the top of the page, and on the resulting page download and install PaintShed. It's an 81MB file to download. PaintShed is needed for quite a wide number of rolling stock items so it's a very useful addition.
The TRS2004 Manual is found by double clicking the desktop icon. When the Trainz menu window opens, look for a button for the manuals. There are a few manuals to go through.
I fought the game for a week or two before I found those, and they really made life easier. For one thing, I didn't have to manually level the ground to smooth the track.