Most networking cabinets and enclosures on the market today are termed "relay racks" in marketing literature. I received an email advertisement from a vendor for one today and began wondering why they are named as such. I've never seen any sort of relay installed in one. Wikipedia has this picture of telco relays in their article on relays. Is this the origin of the term?

If memory serves... its because they are used to "relay" a signal.

I'm going to guess it's because they were originally created to house telephone relays in the telephone company's switching rooms. They had hundreds of them.