If I assume you are talking about square post hole rack rails, the way to remove them is less than obvious and almost invisible when they are installed in rack.
There are 2 ways to remove them.
- Access above/below:
In this case there is a spring loaded tab that faces the outside of the racks that can be pressed in if you have smallish hands or wide racks. Tackle the back mount point first, push in the tab then pull on the sliding portion to release it, then repeat the process on the front.
- Fully populated above and below:
The same technique applies but in this instance you should use a small, thin flat bladed screwdriver and insert it into the hole in the middle of the rail mounting area. You should only insert it approx 1cm into the hole. Gently lever the spring loaded tab towards the server itself, i.e. move the handle towards the outside of the rack. It should not require much force. Then as above, pull the rail towards you.
The imgur gallery here (http://imgur.com/a/kqA83) shows the spiring loaded tab and the access hole highlighted in red in the first and second images respectively. In the second image it is orientated as it would be for a LEFT hand rail in the rack.