What is the rail in the middle of a rackmount 4U server chassis?

[phobostau]

Okay, so as far as I can tell, it's not a card retainer (for extra support on PCIe cards) because it's too far forward and there's no where else to mount it. So what the hell is that thing for? Support against torsional stresses? A cable management system? It sits directly on top of the motherboard, and I can't find a single manual that explains its purpose.

And what are those spring loaded clips for (circled)? I can't figure out what these do.

 

[alaricljs]

That rail across the chassis makes it stronger and reduces torsional flex. The finger looking things with the black tips are PCI(ex/e/X) card hold-downs. The screw in the ATX IO bracket holds down one end, those fingers hold down the other.

 

[phobostau]

Yeah, that makes sense.