This is a mixture of chipping at the dry mud, then streaking it as per Martin's suggestion:

and the other side (there's only one muddy wagon at the moment). Looking at these, It might be worth doing a little more to get the vertical stripes of thin mud (presumably waterborne) around the thick streaks.

I also fixed some underframe damage to one of the wagons and took a couple of new pictures of the first two since there was a bit of sun this lunchtime. You can see the effect of drybrushing the underframes, which wasn't really visible on the earlier pictures - I think it was worth doing.

All in all I'm fairly happy - in the sun, they seem to match up reasonably well to the pictures (also mostly sunlit) which inspired them. I also had a go at an interior last night (not finished yet) using a mixture of stippled tube acrylics and Mig powders. It looks promising (the tube acrylics allow a fantastic rust texture, which then acts as a strong anchor for the powders) but needs some more work before it's ready. More soon hopefully.