The GCT concourses lead to the "train shed," one of the largest underground structures in Manhattan. The train shed has 30 platforms on two levels. Ventilating the train shed has become increasingly difficult over the years; widespread use of air conditioning equipment has increasingly added waste heat into a facility designed long before the age of air conditioning. Ventilation is currently provided by sidewalk grilles and a few small vent shafts.
Temperatures at ground level in the train shed are typically about 15ºF above ambient during the summer months. Improving passenger comfort is but one of several reasons why efforts are being made to improve ventilation, although passengers generally only move through the platform for a few minutes before they enter the air conditioned trains. Another concern is improving the comfort of the MTA employees who spend even more time in the train shed. A primary concern is increasing the service life of apparatus on the trains that must contend with high ambient temperatures.