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Athearn Double Door Boxcar Athearn's 50ft Double Door Boxcar...
Athearn makes a good 50ft double door boxcar.  It is not perfect for a Chessie car, but it is close enough.  You can get them for around $6-7 each.  I got a huge deal and got two undecorated ones for $5.75. Even at full price, these cars are well worth the money.  Having some on your pike is a must.

Below is a Dean Heacock pic of two real double doors.

To get these cars completely accurate would require removing the walkways and filling the holes.  I didn't want to do that with mine.  Also you will need to make the two doors different sizes like the pics above.  I left mine as is.
Mine came undecorated and I painted and decaled them myself.  I am uncertain as to which schemes, if any, Athearn puts out for Chessie, B&O, C&O, or WM.