In 1988, Caterpillar produced their 25,000th elevated drive sprocket track-type tractor, a D9N. The {{convert|405|hp|abbr=on}} D9R replaced the D9N in 1996. The {{convert|410|hp|abbr=on}} D9T replaced the D9R in 2004. The D9T has a low emissions ACERT diesel engine. The current model is the D9T, however older models such as the D9R, D9N, and D9L are still commonly used. The L, N, R and T models of the D9 are visually very similar, differing primarily in the design of their internal systems.
The D9L was replaced by the {{convert|520|hp|abbr=on}} D10N in 1987. The {{convert|370|hp|abbr=on}} D9N replaced D8L in 1987. The main difference between the D9T and the D9R is the installment of the new [[Cat C18 ACERT]] [[engine]]. The D9L is the most powerful D9 in history, with a flywheel power of 460 hp. The D9L is also the heaviest D9 in history at 130,000 lbs.
The D9's primary working tools are the blade, affixed to the front and controlled by six [[hydraulic]] arms, and an optional , which can be attached to the back. The blade is mainly intended for earthmoving and bulk material handling: pushing up sand, dirt, and rubble. It also can be used to push other heavy equipment such as earthmoving scraper pans, and in military applications, [[main battle tank]]s. The dozer [[blade]] usually comes in three variants:
