The new HP Server Chassis have increased in width to the point that some of the models are no longer able to provide slide rails. The manufacturer’s like HP, Dell, and IBM have shoved about as much electronics as they possible can into the 1U and 2U chassis. This has forced them to make the chassis as long as possible. Apparently HP has found a few more millimeters of space by increasing the chassis width.
This chassis width can prevent the HP servers from being installed in 2Post Relay Racks using the HP Rail Kit. The solution to this issue is using a 3rd Party Rail Kit from RackSolutions. There are several Rail Kits available for the new HP chassis design.