This is the kind of trail that takes things literally. It finds a contour and then follows it, remaining at a fixed elevation, like those contour lines you see on a topographic hiking map.
By necessity, because of those restrictions, a contour trail cannot actually go anywhere, or even exist in anything resembling ordinary reality.
On average terrain the elevation simply changes too often, and by too much, so at best a contour trail could only loop back on itself, if it was really, really short. This is all right per se, but never gets better than boring. If you like walking in tight circles around a lone tree, then this might be your event. (Net elevation change per lap: zero.)
And the sort of terrain where a real contour trail might be possible is actually so rare that most such trails are in fact artificial, and are used only for synthetic activities. Think of the Daytona International Speedway, for example. Or think instead of the La-Z-Boy 500, which is not only slower and safer than NASCAR events, but even less interesting, which makes it great for napping to if it's on TV.
Want another example? Sure, why not?
Imagine the Bonneville Salt Flats in Utah, "a place so flat you seem to see the curvature of the planet, so barren not even the simplest life forms can exist". The description makes it seem uncannily similar to the typical office environment yet it is so different when you actually get there, being a place where you may experience "the passing thunder of strange vehicles hurtling by on a vast dazzling white plain". Instead of some idiot stinking up the place with microwave popcorn every afternoon at 2:35, and playing country rock on his radio in the next cubicle all day, every day. ("Well it doesn't bother me.")
Both those environments are so infinitely two-dimensional and sterile and featureless and brain dead that there is nothing to contour around anyway. And they are no good for hiking, which is what we're really interested in, so screw them all. Let's be wild and dangerous and go cross-country. How about it?