xanthra - I knew what you were talking about. To me tracked vehicles do pretty much the same thing with alot less labor.
Umm... hate to break it to you, but how does the military transport their tanks and heavy guns? By rail : ) Rails have always been the most efficient way to transport heavy cargo.
The problem there is the the laying of tracks in front of the city instantly detracts from any extra efficiency the system would have had otherwise. The effort required to lay tracks of a sturdy enough nature to support the city, fast enough for the city not to have to slow down, could be instead funneled into the construction of a fast, efficient engine and track system.
The efficiency of rail as a system for transporting cargo also relies on the fact it's a closed system. Once the tracks are laid, which is a process which can take many, many months for the distance the transport would move in a day, the transport can travel along this closed loop as fast as it needs to, back and forward. It's a system that doesn't lend itself to the flexibility required for a hunter city.