The Mark IV Tamper is one of the most common tampers in the industry. Developed in the late 1990s by Fairmont Tamper (now Harsco Rail), this machine replaced the very successful Mark III Tamper. The machine can be identified by it's unique wedge shaped frame. Virtually every class one railroad, with the possible exception of BNSF uses this machine, as well as many transit lines and contractors.
All tampers work under the same principle - to "tamp" the ballast under and around the railroad ties to increase stability of the roadbed. The main working heads are directly in front of the cab, and include "spades" that are shoved into the ballast on either side of a railroad tie, both inside and outside of the rail. These spades are connected to vibrating motors, which when inserted into the ballast, vibrate the individual rocks in place to lock in the ties. Often the machine will raise the roadbed several inches at the same time to help relevel the track. This is done with a lifting and clamping mechanism.
So take a look next time you see some MOW equipment - good chance you will notice a Mark IV Tamper!