The Baltimore branch of a national relocation services company will become the first tenant in a 1,000 acre office and industrial park being built on Baltimore County's largest underdeveloped chunk of commercial land.First Industrial Realty Trust Inc. one the developers of the $500 million Baltimore Crossroads @95, said yesterday it has signed a lease with Alexander's Mobility Services for 130,000 square feet of office/warehouse space and will start constructing the single-story building later this month.