That truck had obviously been retrofitted to run on the train tracks. And the train tracks seemed maintained. So was there some other town somewhere that Bartertown did importing/exporting with?
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I found this comment from this site
http://nitcentral.philfarrand.com/discus/messages/8/18581.html
So, Jedediah and his son have a blockade at the end of the tracks? Were they expecting a train to come by so they can rob them? Do trains come that way very often?
I guess:
At the beginning of the Civilization of the Living (Bartertown), the Truck/Train hybrid was ordinarly used. Then, after some time, it was converted into the energy core of Underworld.
As simple as that, Jedediah Junior heard the noise of the Truck/Train hybrid running on the tracks and decided to rob whoever was aboard. Not complicated.
"Mad Max Beyond Thunderdome" is one of the most visionary movies of all times and a massively underrated masterpiece.
MAD MAX MOVIE TIMELINES - Continuity explained!
http://madmaxtimeline.blogspot.com