Dell S3048 Guide De Mise En Route page 14

Figure 5. Attaching the Switch Rails
2. After you have installed both switch rails, line them up on the previously
mounted Ready-Rails and slide the switch in until it is flush with front of
rack. About 3 inches before you fully insert your system, the rail locking
feature engages to keep the switch from inadvertently sliding out of the
rack and falling.
NOTE: Do not the use the mounted Ready-Rails as a shelf or a
workplace.
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