There are a large variety of 50 foot boxcars in service today. In Nscale, we are very fortunate to have both Micro-Trains and MDC/Roundhouse producing good models which can be used by CSX / Chessie modelers. Here are a few.
C&O Chessie
This MDC Roundhouse model is one of the many IPD style cars that went into service in the 70's and 80's. This car has 1025 couplers, and no other modifications (yet).
CSX BIG BLUE
The CSX Big Blue model is made by LBF. The prototype is a Gunderson Car, part of an order of 220 Plate F cars. BIG BLUE cars are used for newsprint service. A great article on these cars can be found at http://www.intellitrans.com/intelli2/img/news/railwayage.pdf
My model is molded in blue plastic with a silver roof, and comes weighted. The under frame details are good, up to MT standards. The only real complaint I have with this thing is that the roof comes off too easy .. Overall .. at around $15 its not a bad model, and will look good servicing your Walthers paper mill.