Perhaps no weathering technique is more controversial than duplicating the graffiti found on many trains today. Some call it art, some call it vandalism. Like it or not, it is a part of the modern railroad scene that many try to duplicate. Many who do argue that they put it on the model because it's prototypical, not because they agree with it.
Graffiti had been around for decades before the brightly colored spray paint pieces that began appearing on train cars in the mid 1990s. Railroaders used chalk or grease pens to mark routing instructions in a sort of railroad shorthand on the sides of cars to help sort cars in yards. Hoboes also had a symbolic language of their own that appeared on train cars and around camps.
If you choose to add markings to your trains, there are three easy methods available. Having a good picture of the prototype you want to duplicate is always a good place to start.