Dell EMC PowerSwitch S4200-ON Series Setup Manual page 15

Figure 7. Switch rail attachment
2. After you install both rails, line them up on the ReadyRails. Slide the switch in until it is flush with the front of rack.
About three inches before you fully insert your switch, the rail locking feature engages to keep the switch from inadvertently
sliding out and falling.
Figure 8. Front rack installation
S4200-ON Series Installation 15