Norfolk Southern Corp. will take over most of a 12-story building in Roanoke and add three floors to its Atlanta office building to accomodate employes transferring from the former Southern Railway headquarters in Washington.

Norfolk Southern, headquartered in Norfolk, was formed last June by the merger of the Norfolk & Western Railway and Southern Railway. N&W and Southern operate as separate subsidiaries of Norfolk Southern, but some functions are combined. Southern's operating headquarters will be in Atlanta and N&W's will remain in Roanoke.

About 150 operating personnel for Southern will be moved to Atlanta. The addition to the Southern office building there is expected to be completed in about a year.

A group of 200 marketing and other personnel will be moved to Roanoke, where they will work in 10 stories of the former Colonial American National Bank Building. The building will be renamed the Norfolk Southern Building and is expected to be ready for use this summer.

Nearly 100 corporate personnel will be moved to the headquarters in Norfolk.

Some public affairs and sales employes will remain in Washington, but for the first time since the railroad's founding in 1894, there will not be a Southern Railway headquarters in Washington.

Other recent developments in commercial realty in the Washington metropolitan area:

* E-Systems Inc. has leased about 8,300 square feet of space at One Flint Hill at 10530 Rosehaven St. in Fairfax. The aggregate rental value of the lease is about $600,000. E.C. Ernst Inc., an electrical contracting firm, has also leased about 22,000 square feet of space at One Flint Hill. The aggregate rental value of the lease is over $1.8 million.

* Wallach & Associates, an executive search firm, has leased 4,300 square feet of space at The Atrium Building at 6101 Executive Blvd. in Rockville.