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Fortunately, Roundhouse makes a great Railgon gondola kit.  The only problem is the B&O number it comes with is bogus.  A quick painting and decaling can give you a nearly perfect B&O Railgon gondola.  The #351038 is actually a correct number for a C&O Railgon gondola.  To make that car just paint over the "B&O" and add white "C&O" letters in their place.  I wanted this Railgon to be a B&O car, so I chose #350477 as my car.  It appears that all the C&O cars were repainted with white "C&O" letters, this was not the case with the B&O cars.  Some B&O's had white "B&O" letters, some yellow.

Below is a John Whitmore pic of the exact gondola I am modeling.

                                                                                John Whitmore photo
How to:
1.  Choose the exact railgon you want to do.  I chose #350477 since I had a picture of it.  See above.  Plus I like the look with white B&O letters and a yellow number.  I think that look really stresses the fact that it is a secondhand car.
2.  Take the shell of the Roundhouse kit and tape off around the B&O letters.  See pic below.
3.  Paint over the letters with flat black paint.  You can lightly sand the factory applied letters if you want before painting, but I didn't and you can't tell they were ever there.
4.  Spray the whole gondola with gloss coat.
5.  Apply B&O in white letters (I used extras from a Herald King Careful Car Handling caboose decal set) and 350477 in yellow numbers (I used extras from a Herald King Chessie Boxcar decal set).
6.  Spray with dull coat.  Add the brake wheel.
7.  Weather the outside and inside heavily with rust, dirt, earth, roof brown, and black streaking and oversprays.  These cars got beaten up pretty bad, so don't be shy on the weathering.
8.  The insides of gondolas are not clean.  Add some diluted white glue and sprinkle in some fine dirt or sand, also add a spare piece of scrap wood or any little thing to look like trash left over.  I crushed some rust from an old piece of metal and sprinkled that in too for added texture.
9.  Spray with dull coat to seal it all.  Add the underbody details, trucks, and couplers.  Weather those parts too.
10.  You are done, enjoy your car.

Below is a pic of the Railgon out of the box.  I had already started masking the other side when I took the pic.

Below is a pic of the Railgon masked off for painting.  I covered the rest of the car with a paper towel to prevent black paint from getting on the other lettering.

Below is a pic of the Railgon painted over and the gloss coat is applied.  Up next, decals.

Below is a pic of the Railgon with the new B&O letters and numbers.

Below is the finished car.

Below is a pic of the top of the car.