On side streets and under the L, they sat like discarded props from an ’80s post-apocalyptic B-movie. Metal screens covered or entirely replaced windows. Blanking plates obscured headlamps. Rust and dents marred just about every surface, and all their original markings had been painted over and tagged with cryptic stencils. Despite having the look of burned-out hulks abandoned to squatters and street gangs, these field support buses were maintained enough to move around the city of Chicago—though for what purpose it's not immediately clear.

We came across more than a dozen photographs of field support buses in Northwestern's Ronald J. Sullivan collection not long ago while searching for images to include in our Carspotting series. Most of the Sullivan photos, taken over the span of several decades, focus on commuter rail, city buses, and coaches in and around Chicago. They capture not just people bustling about the city but also slices of traffic and less-explored streets. Many people may overlook these mundane and routine details, but historians use photos like these to glean important documentary information of how people lived.

Which brings us back to these mystery buses. Most of the photographs featuring these machines date to the 1990-’94 period (though this may be a function of Sullivan's subject selection more than the prevalence of field support buses on Chicago streets). All the buses appear to be removed from regular customer service but they once served a purpose, as shown by the power cables strung between them and nearby telephone poles, the locking doors, and even the doormat in the top photo. They've all suffered some sort of damage—not enough to render them unroadworthy, but enough to make a passenger question why they'd get on that particular bus.

Sullivan called them "field support buses," a term we don't see in widespread use, but one that seems to suggest the buses were used to service Chicago Transit Authority (CTA) infrastructure like tracks, switching equipment, transformers, and the like. A sort of inexpensive construction trailer made from the shells of retired buses. We have a request in with the CTA to see if our hunch is correct, if this was a widespread practice among transit agencies, and if these field support buses are still in the fleet. If that isn't the case, tell us in the comments below what other purposes you can imagine these buses serving.