Complex sentence: In May 2010, the MTA leased the air rights over the railyard for 99 years to a joint venture of Related Companies and Oxford Properties Group, which will build a platform on which to construct the buildings.
Split into simpler sentences: In May 2010, the MTA announced it had signed a contract with a joint venture of Related Companies and Oxford Properties Group. The MTA will lease the air rights over the railyard for 99 years to the joint venture, which will then build a platform over the rail yard on which it will construct the buildings.