Here I am last week touring Grand Central Station.  I was on my way to buy some books when I decided to get off at 49th Street to do some tooling around first.  I think the station at 42nd and Times Square is more central than Grand Central, when speaking in terms of the New York City subway system.  However, Grand Central is a terminus for the more far-reaching commuter lines of  Metro North, in addition to a number of subway lines.  Check out the standard Grand Central Station Wiki entry.