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Railroad freight cars never get washed.  Locomitives sometimes do, but they too get a coat of "road grime" on them.  Road grime is seen in the pic below of a typical Chessie boxcar.  The coat of dirt is relatively even across the car.  This shows that the car has been in use and not brand new.

How to:

To apply road grime, use a dark grey color thinned 1 part paint to 10 parts thinner.  Coat the entire car and see how it looks.  You should notice only a slight change in the appearance of the car.  If you are happy, stop.  If you want the car to be "dirtier" then add another coat.  Keep adding coats until you are happy with the look.

Next change to a very thinne tannish color.  I use a color called "dirt" (appropriate isn't it).  Overspray this color in a horizontal stripe along the bottom 1/4 of the car.  This will represent the most recent grime.  This should be a light coating.  Don't over do it.

Remember that most cars are moderately dirty.  Few are pristine and few are completely filthy.  Don't weather all of your cars in extremes.

Below is an example:
The top Bowser 100T Hopper is fresh out of the box.  The bottom one has been oversprayed with just two coats of the very thinned dark grey.  See what a difference it makes.  The bottom car looks like it belongs on a railroad, the top one looks like a toy.