I've used decals for graffiti, but (in my experience) they come out a little dull.
I've had the same problem. Because most of them are ink jet printed on clear film, they aren't as opaque as they need to be. They can only be applied to light colored cars.
There may be a way around this, but it isn't easy. Paint the area under the decal white before applying it. This should help make it pop. I have yet to try this technique, but the theory seems sound.
I have made some decals by isolating the graffiti from photos of real trains using editing software. I plan to print them on white film and trim them closely.
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In the real world, the railroads have three basic rules regarding graffiti:
- Don't cover reporting marks and car numbers. Most artists know this, and abide by it.
- No profanity. Again, true artists comply with this. When found, it is usually removed.
- Do not do it near live tracks (safety).
After that, the railroads don't care, so why should you? Get over it.