Ken, the older style of Aussie livestock trailers had timber sides, but later versions are similar to those on the B Double. They are more resistant to damage, and easier to repair than timber, and they're lighter. The vehicle you are talking about is a three dog road train, in other words, a prime mover with three trailers, known as dogs. These are very common in outback areas of Australia, but are restricted in more built up areas. Many larger country towns have an area set aside on the outskirts of the town where the road trains can be broken up into individual trailers unless the train is continuing farther on. In most country towns where road trains are seen, the main highway through town is nearly always quite straight, so road trains can safely travel through.
Overtaking a road train can be a very interesting exercise, especially when towing a caravan. It needs to be done with extreme caution, and in the right place.
I've attached 2 photos, one of a livestock road train, and the other of a bulk tipper with 2 dog trailers followed by an attached B Double.
Peter.