Keilim, Chapter 24, Mishnah 2. This chapter takes one object after another and divides it into three forms, and for each form it tells us which kind of tum'ah it can receive. Here the Mishnah turns to the wagon.
The Mishnah opens by counting: "Shalosh agalot hen." Wagons fall into three categories, and the halachah assigns a different status to each of them.
A wagon built like a chair
"Ha'asuya kakatedra, teme'ah midras." The first wagon is fashioned in the shape of a seat: it is closed in on three sides, with a back and with armrests, exactly like an ordinary chair, only that it has wheels beneath it. Since it was made for a person to settle himself into it, it serves as a seat, and therefore it is susceptible to tum'at midras, the tum'ah that comes to an object designated for sitting or lying upon.
A wagon built like a bed
"Kemitah, teme'ah tum'at met." The second wagon is shaped like a bed: a long open box, the sort of wagon in which merchandise is carted about. It is certainly a utensil, and as such it can become tamei through corpse tum'ah. But it cannot receive tum'at midras, because it was made to carry goods and not to carry people, and midras applies only to something designated for a person to lie or sit upon.
A wagon for hauling stones
"Veshel avanim, tehorah miklum." The third wagon is the type used for hauling heavy stones, and no form of tum'ah takes hold of it at all. The reason is its construction: its floor consists of planks laid side by side, and the openings left between one plank and the next are wide enough that a pomegranate could drop right through. Earlier chapters of the masechet established that a utensil breached by an opening of such a size loses its capacity to become tamei, so this wagon stands entirely outside the laws of tum'ah.
So the single word "wagon" covers three very different objects: one that is really a chair on wheels and takes midras, one that is really a container on wheels and takes corpse tum'ah alone, and one whose very structure of widely spaced boards keeps it outside the laws of tum'ah altogether.