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Probably published late 1970s or early 1980s. A large group of people are organized into a small city that is in a huge contraption that has to keep moving around the earth after some apocalyptic event. They might call the conveyance "Earth City." They have to keep pulling up track behind and placing it in front to proceed.
They are following electromagnetic pulses in the earth's crust that provides motive power for the thing.
The direction in which they are heading is called "up future," whilst the direction from which they have come is called "back past." Any ideas would be much appreciated.