visited tourist sites. 750,000 visitors a day. open in 1913, the four-faced clock makes grand central instantly recognizable. now this iconic 20th century american landmark is jumping into the 21st century. certainly the temple of mass transit in the united states. we built the whole new railroad terminal right underneath. reporter: for 15 years, crews have been dynamiting and boring under the existing grand central. with a staggering price tag of $11 billion, it s the biggest expansion since 1910. in new york, transit is like air and water. chairman and ceo jan gave us a behind the scenes look. marble walls. murals. and quotes from jerry seinfeld , jay-z and alicia keys we are going deeper and