The track on an automobile (and I assume a wagon or carriage) is the distance between the centerpoints of the two wheels on an axle. Do you happen to have a picture of the page for context? I have a few theories but I don't want to speculate without more to go off of.

EDIT: Seems one of my theories was correct. I found this 1897 Sears Roebuck & Co. Catalogue which states:

READ THIS CAREFULLY BEFORE ORDERING and save mistakes and misunderstandings. Different widths of track are used in different localities. Always state whether you want a wide or a narrow track wagon. Our wide track wagon is 5 feet and our narrow track wagon is 4 feet 6 inches from center to center of tire.

So basically you wanted to make sure your wagon would fit on the roads where you lived. Luckily they were both the same price, as stated at the bottom of the page:

IN ORDERING be sure to state just what is wanted, say whether you wish wide or narrow track, 4 feet 6 inches from center of tire to center of tire, or 5 feet wide, center of tire to center of tire, the price is the same.

Now I'm not an expert on wagons by any means, but I would assume it would be mighty uncomfortable to use narrow track wagon on a wide track road where the ruts by previous wheels would be ~5 feet apart, as your wheels would constantly be at risk of going into a rut. Not only would this be a bumpier ride than usual, but it could also risk breaking a wheel if torqued the wrong way. Imagine something like this, but with narrower ruts. In a narrow wagon on a wide track you would constantly be either tilted or going in and out of the ruts. A wide wagon might not fit on a narrow track, creating the same issues.

Wagons running on unmetalled roads quickly create deep ruts, and your wagon needs to match the size of the ruts, almost like a railcar needing to be of the right gauge.