As this "show is outside", and the units are operational, I am after an overall effect rather than creating individual weathered masterpieces. Not that it is wrong, it is simply not my goal so I keep it simple. I use oil washes and dry pigment either together or stand alone.
Oil wash weathering is easy to do with artist oils, thinner, a brush, thinner, and a prototype photo for reference.
For base coats I focus on 3 colors - dark grey (or black), burnt umber, and burnt sienna. You should use the colors that reflect your model railroad's geography.
Squeeze some of the color onto the working surface (it has the consistency of toothpaste or caulk). There are 2 ways to apply the color. As a wash, wet the brush in thinner, mix with the oil (thin), and use vertical strokes dragging the color down the side of the car. Repeat if needed.