GO FOR IT!  It can be daunting to take a nice model and feel like you might ruin it. In chemistry terms, it has a “high activation energy”, but the rewards are many and the risks actually few. Some suggestions beyond what others have posted.

1) Start with Chalks. If you use plain chalks, they are easy to wash off. In addition, when you clear coat, they get much less intense. Chalks with adhesive, such as Bragdon’s or the ones I use, Doc O’Brien's Weathering Powders (Micro Mark https://www.micromark.com/Doc-ObrienS-Powders) are a little harder to remove but will come off too.

2) I’m a little crazy, but my 1st weathering attempt was a P2K PRR HH1 (N&W Y-3) 2-8-8-2. Weathering a $400 locomotive is enough to keep you focused! You might want to work on a freight car 1st ;)

3) Acrylic washes are also easy to work with. I do them over acrylics, so I have to be careful when removing. The most common approach is something Jim Six called “Q-Tip weathering”. Easy-peasy! Take a nice thin grunge wash, and brush it on the car side. Now, using a cotton swap (e.g. Q-tip), gently stoke down the side of the car, removing most of the paint. Keep changing swabs to keep removing paint. What is left will be in the shadow of rivets, seams, etc. Do one half of one side at at time. Let dry and admire (then add chalk marks…)

Now, there was the time I got too heavy handed and airbrushed an all over “wash” on a RC express X29 (for the complete grunge covered look) and it totally sucked (too much paint, blotching, “flowering”, etc). So I literally poured 90% isopropanol on the car side and started scrubbing with Q-tips. The wash sort of came off, as did some of the lettering. It actually looked beat-to-hell great! Then it dried with a white filmy look (UGH!). Thinking back to advice here, I wondered if a clear coat would get rid of the haze (I’m tempted to call it an “alcohol haze” but then folks might comment on my drinking habits!). Rescued. Now this is one of my favorite weathering jobs… happened 100% by accident. Which leads to #4

4) Make lemonade out of lemons. Did you totally screw it up? Fine!  Strip the car, repaint, decal, and do it again. Or maybe too much weathering? Gently remove the acrylic wash with isopropanol and a Q-tip. Not enough weathering? That the easy one, just add more! ;) 

5) As Jim Betz noted and as I have presented. Weathering is due to the action of nature and man on machine. Think about all the sources of weathering (not just rain, but road dust, other cars, mishandling, etc…) and how the job of that particular piece of equipment affects its weathering (My next “EXTREME weathering project will be a Jordan spreader)

6) Finally, it isn’t necessary to make every car a work of art. Here are two quick “fleet weathering approaches”. These can be chalk or acrylic washes

Boxcar

- Fade the sides - lightly cover with a thinned color that is the same or a lighter version of the body color

- Fade the trucks - grit blast the side frames (protect the bearings with tape)

- Fade the underbody - a light coat of grimy or faded black, or darkish grime colors so show road dirt

- Fade the roof - a light coat of grimy or faded black to mimic the soot of the steam era

Tank car (black)

- overall fade with tarnished black (remember, if it is dark, make it lighter!)

- grimy or oily black around and under the dome (oil drips)

- fade the trucks

- thin rust wash next to the tank bands (to simulate their slipping as the tank expands and contracts)

Weathering is a nearly endless subject, with so many cool approaches. I typically use 2 to 3 different approaches, including washes, chalk, and airbrush to weather an individual car. For example, there are metallic paints that can then be oxidized to generate REAL rust finishes, or real copper. The more layers of weathering, the more realistic it looks. Military modelers have a LOT of great ideas and have been doing this a long time. One recent You-Tube video of a German tank build included something ridiculous like 80 HOURS of weathering! But don’t panic, 20-30 minutes can get you a lot of nice effects too!