CRG West, a company that develops and operates IT data center facilities around the country, is reporting steady customer growth at its Terminal Annex and One Wilshire locations in Downtown.

Both properties in recent years (as with several office properties in the Financial District) have been built out as some of the most sophisticated telecom buildings in the country, housing floors of not people, but communications and computing infrastructure for a variety of companies.

Last week, CRG West announced that increased demand has lead to an expansion at their Terminal Annex site, filling in more of the 450,000 square-foot shell already supporting virtual companies like Slide.com and eHarmony. AppNexus, a provider of cloud computing services, has expanded their footprint to nearly 2,000 square feet.

The historic building is located across from Union Station and was originally constructed in 1939 as the United States Post Office’s central mail headquarters.

While wire bundles and racks of circuit boards may seem like a waste of prime office space, it’s a profitable use for a landmark property that might otherwise sit vacant.