Theres not really any `Art' to it, I think there are only two `rules' when weathering wagons, or indeed any models, and even these are not by any means fixed rules, merely guidelines that can help you to produce more realistic models.

Rule 1: Work from Photographs! Try to work from a photo of the exact vehicle you are trying to make, If you cannot find a photograph or image of the exact vehicle, look for something in the same group, class or type and use it as a reference to see which bits were worn, rusty or dirty on the real thing. This is much better than trying to guess, When modelling historical prototypes a Black & White image is better than none and can often show finer detail than early colour photographs.

Rule 2: Apply weathering in THIN coats! Real wagons are mainly one colour when new, but that doesn't last for long, especially when modelling from the days of steam. All railway vehicles, especially goods stock, rapidly accumulated dirt from the harsh environments in which they worked, the various `layers' of grime being washed into corners & seams between boards by the rain, which also `cleaned' some of the muck off the roofs & wagon bodies giving them differing `shades' and `hues' of the original colour. There is also general wear & tear to think about, things like worn edges on foot steps & below doors.