When I was asked to weather this TruLine Trains caboose, I looked through a number of photos to use as reference. I didn't try to copy any exactly like a particular prototype.
I first faded the overall colour, then patched over imaginary graffiti all along one side. I then patched in the reporting marks, and added some graffiti to the other side. The shine of the black roof was flattened down, and rust blisters and streaking down the sides from rust and grime runoff was added.
My favourite part of the project is the rusty expanded metal grating that I put over the side windows. That should stop any HO scale vandals from throwing any ballast stones through them.