The 'Rut Iron' (or 'Track Iron'): Old Golf Club
Once upon a time in golf's history, the lands on which golfers played were public lands that were frequently criss-crossed by townsfolk, either walking or riding in (or pushing) carts or wagons. The wheels of those carts and wagons left impressions in the ground — ruts, or tracks. So what would a golf club designed specifically to extricate golf balls from wagon wheel ruts or cart wheel tracks be called? A track iron or rut iron.
And that's what the rut iron was: It was an iron-headed golf club whose head was very narrow, rounded on bottom, with a small, concave face, designed specifically to get down into those narrow wheel ruts and dig a golf ball out. The rut iron was a specialty club, used by most golfers who had one for that singular purpose.
The March 1901 issue of Golf Illustrated magazine refers to "the ancient rut iron" as a precursor to the niblick, indicating that rut irons were both very old golf clubs and had already faded away from most golfers' repertoire of clubs by the 20th century.
Why would rut irons fade away? By the last few decades of the 1800s, golf courses were starting to become places specifically designed for the use of golfers (as opposed to public land adapted to the playing of golf by enterprising linksmen). And that meant the holes started being closed to wheeled traffic. If wagon tracks and wheel ruts begin disappearing from the specified playing areas on golf courses, then the club designed for golfers to use if their ball went into such tracks and ruts would naturally also begin disappearing.
Note that in addition to rut iron and track iron, these clubs were (or are) known as rutters, rutting irons, rutting niblicks or iron niblicks. "Iron niblick" and "track iron" might have been the most common name used in their heyday. "Rut iron" may be a term that came later and was applied more retroactively.
In his book Tommy's Honor: The Story of Old Tom Morris and Young Tom Morris (affiliate links), author Kevin Cook writes that the rut iron was one of Young Tom Morris' favorite clubs:
"A forerunner of the sand wedge, the rut iron was made for digging a ball out of cart-wheel ruts. Most links were crisscrossed with ruts — cart tracks barely wider than a golf ball. The rut iron was a lofted specialty club designed to flip the ball out of a rut to safe ground. But Tommy used his rut iron from the fairway. As a Prestwick Golf Club historian wrote, he 'developed the art of playing approach shots with a rut iron, a shot so difficult as never to have been attempted before.' By swinging sharply down with the lofted little cleek, Tommy launched high approaches that stopped or even backed up when they hit the green."Young Tom Morris' time was the mid-1860s to the mid-1870s. The Historical Dictionary of Golfing Terms cites an 1850s reference to the fact that at St. Andrews "the iron niblick is termed a track-iron." But this type of club probably dates to the 1700s.
You'll find references (including above) to the rut iron being a precursor to the niblick or a forerunner to the sand wedge, but the rut iron did not evolve into a different club. It disappeared from the game. You can, however, accurately say that in the family of clubs that includes niblicks and wedges, the rut iron is a branch on that evolutionary tree.
The term has been revived by modern manufacturers a time or two, but applied to clubs that have little resemblance to the real rut irons of the 19th century.