Thanks! I have been playing with some cheap oil paints I bought from the Works and it basically involved painting a black / burnt umber mix stripe down the wagon, leaving it for about 40 minutes, then using a large-ish brush with a touch of thinners to manipulate it.

Not an exact science I'm afraid - I just played with it until it looked right to my eye (as usual, the effect looks harsher in the picture than it does in real life).

Tank you for sharing your technique, looks very effective.