Ray,

  Take a box car you don't really care about and weather it.  Build up the
weathering slowly - taking a day or two between to just look at it several
times a day to see "when it is enough"..  Work from a photo - try to copy
some prototype example ... or the work of someone else that you like.
Do one side of it - then do the other side differently (more practice, less
sacrificial lambs).

  I use acrylic -washes- applied with a brush ... here are some 'basics'.

  1) The roof is almost always more weathered than the sides.
  2) Darker colors on the roof and lighter colors on the bottom.
  3) Cars sit more than they move - a lot more.  So any "streaks"
       need to be vertical rather than horizontal.
  4) Use gravity to let your washes actually move down the car sides.
  5) A final light dusting with an air brush helps a lot - I call this the
      "blending coat" - I usually use a very thin "weathered black" color
      for this but have also used just dullcoat and other such.
  6) Weathered equipment is never "shiny".
  7) Weathering on the prototype is a "process" - with variations depending
       upon where the car has been, how long it's been since it was painted,
       what kind of service it is in (cement hoppers are entirely different than
       ore jennies), etc.  
  8) Be careful not to over do 'special effects' such as bird droppings, rust
      "lines" along the rivets, etc.
  9) I use a combination of "detail painting by hand (grabs and drop steps
      and other metal parts)" and "general effects (washes - usually done
      after the detail items but not always).
10) Rust is a job best done sparingly.

  If you study a photo of a steam era freight yard the first thing you
notice is that "all the cars seem to be the same".  Closer examination
shows subtle differences from this car to the one next to it.  That's
the look I strive for ... said another way "don't fall in love with just
one process/set of steps - variety is the spice of weathering".

  Your first attempts are likely to be 'failures' (that's why we used an
old car we don't care about).  Most of the time it will be due to too
much rather than too little.  Even your worst weathering job will be
better than no weathering at all.  *G*
  Keep your test car around and run it on the layout every once in a
while - to remind you of "where you aren't going".  *W*  And how 
far you've come since you started down this journey.

  Weathering is like the student mathematician who went to see his
girl friend.  First he went half way there, then he went half way more,
then he went half way more again, etc.  He never really got to where
his girl was ... but he got close enough for all practical purposes.
                                                                                                       - Jim

P.S. There are many different 'methods' - I prefer acrylic washes.
        Some guys prefer pan pastels.  Some guys like to do it all
        using an air brush (I consider this to be the least successful).
        In the end you will develop your own 'process'.  Don't forget
        to vary what you do from car to car - such as the shade of
        this coat, how much of a particular coat you use, what order
        you do different steps, etc.