What do you mean by "Improve"? Make the car more reliable in operation, better appearance?
Weight and correct coupler height along with free rolling trucks will make a car perform better. A light coat of Testors Dullcote airbrushed on the car can take the plastic shine away.
There is a list of websites with photographs of freight cars at http://cs.trains.com/trccs/forums/t/138458.aspx.
"No soup for you!" - Yev Kassem (from Seinfeld)
My first attempt to "improve" a car is Bachmann's 56' center flow hopper. Searching the web for info gave me zip. Anybody know where I can see a picture or pictures of this thing? Thanks.