I decided to purchase an American Models Diesel Sound Boxcar. It's a 50 ft. boxcar; it scales out, but looks bigger than I expected. I have a couple K-line boxcars that also look big, but coupled together don't look so oversize.
I think the sound is pretty good, and sounds very good at idle. The "ramp up" is adjustable to come in when you want it to; recommended is at about 50% power. The horn and bell are accetable, but not as good as on my Lionel AF U33.
Bottom line: I now have diesel sounds with any of my vintage diesels or my AM Trainmaster for a cost less than $100 after subtracting the cost of theboxcar.