Gimp has a plugin to make Bump maps Brian, thats what i messed around with. I took my normal brick texture and made a bump map from it.
So for this your going to need texture of some corrugated metal to make a bump map.
Then you need to use something like the TrainBumpSpecEnvMask.fx shader , put your normal texture in the normal slot and your bump in the bump slot. You may need something in the enviro slot too but its a while ago i experimented with this so can't really remember .
This was my experiment a while ago. With and without bump. The downside is the bump only shows in light. But thats ok if its a building roof outside.
RailWorks 2014-01-06 19-16-22-19 by christopher.baker9, on Flickr