The new Dell Universal Rail Kits will fit into almost any Server Rack available today. The one exception is a Server Rack that has a C channel shaped post. This type of post will cause an obstruction with the rotating assembly of the rails.
The solution to this obstruction issue is to replace the rotating assembly on the Universal Rail Kit with a simple mounting flange. The most common servers that have this issue are the PE1950, PE2950, and the PE2900.
This Dell Client tried to install a PE2900 into a Server Rack with a C shaped upright. He was unable to do it and got so upset that he made a YouTube video to demonstrate his issue. He ended up bending his rack to install the rail. This compromised the structural integrity of the Rack and was completely avoidable with the use of the Rail Ear Kit: http://www.youtube.com/watch?v=ook7ly4hb0I
This video demonstrates the obstruction issue and how easy it is to replace the rail assembly with the Rail Ear Kit: http://www.youtube.com/watch?v=b8teyeGtZGg