I found cable trays at
http://www.revitcity.com/forums.php?action=viewthread&thread_id=8785
they are free and they seem to work pretty well. They are not as automatic as ducts but you can modify them to suit at least. Revit City is a good site so have a look around while you are there. You need to search in the Forum area for things like this on the Revit sites like RevitCity and RevitFactory.