Crawlers use a modular design, with various kinds of cars that can be hitched together according to needs.
A crawler consists of alternating cars and wheel units, with a drive cab at the head of the train. A fully-loaded crawler can easily exceed half a kilometer in length.
Universal Power Unit (yoop)
Yoops power the crawler. A yoop is little more than a pair of giant wheels on an axle, together with power and drive machinery, and universal hitches front and rear.
The wheels are seven meters in diameter, with a broad sprung mesh circumference. The mesh design provides traction, and spreads the weight. The large diameter provides clearance and allows the vehicle to navigate rough terrain.
Power is from standard fusion cores mounted above the axle.
The hitches are universal joints with a powerful network of hydraulics that actively manage the hitch position. Because the crawler's cars are suspended between pairs of hitches, the hitch has to support and steady the attached car while allowing the right degree of movement to make turns and adjust for the terrain.
Finally, the yoop supports a docking tunnel that links the cars on either side and allows people to move along the length of the train in a sealed environment.
Cars mostly follow a similar body plan, a box twelve meters wide and twelve long. The width is constrained by the need to fit through standard sized entrance tunnels into vehicle garages. Lengths can vary slightly.
Cars have standard hitch and docking mountings front and rear, to attach to a yoop. Because all the power comes from the yoop, cars can be simpler in design and easier to maintain than if they had to provide self-contained services.
Beyond the basic plan, there is considerable variety in car design. Crew cabs provide sleeping and living accommodation for up to fifty people – big enough to support a typical harvesting crew out in the field. General purpose cargo cars cater for bulk movement of goods, while more specialized cars carry harvesting or survey equipment.
Every train has a drive cab at its head. This is a smaller highly specialized car designed to hang on a yoop's hitch without needing a second yoop attached. The drive cab contains the computerized controls needed to manage the complex machinery of a train, as well as accommodation for a small crew.