didn't someone tell me that telco equipment had 40vdc racks once?

[Jonathan Hutchins]

The point of all this is that instead of every piece of equipment having
it's own switching power supply, with fan, you supply the required
voltages to the whole rack from a common pair of failover powersupplies. 
Each box then gets it's on +12,+5, and -5 (or whatever), and we have one
less component per unit to fail.

Converting the power once then distributing it really does beat
distributing the AC and converting it at each unit, there are savings in
equipment cost, efficiency, and cooling.

Because of this, a 48 volt distribution system doesn't make as much sense,
nor  does it offer as much advantage over a 120/240V AC distribution
system.