The firm writes on their site:
Grand Central Terminal is one of New York’s greatest landmarks and contains perhaps the city’s finest civic space. However, over time it has become a victim of its own success. A building designed to be used by 75,000 people per day now routinely handles ten times that number with up to a million on peak days.
The result is acute overcrowding; connections to the rail and subway lines beneath the concourse are inadequate; and the arrival and departure experience is poor. Added to that, the surrounding streets are choked with traffic and pedestrians are marginalised. The rapid growth of tall buildings in the vicinity has all but consumed the Terminal.