All aboard for David Katzenstein's photography of the hidden parts of Grand Central Terminal

Over 750,000 people pass through the building every day, as they travel to and from work. But it's also a meeting place, a tourist spot and somewhere you can shop or grab a bite to eat – human energy is everywhere.

In 2014, photographer David Katzenstein was commissioned by a corporate sponsor to explore three transportation hubs in Manhattan. Grand Central Terminal was the first he documented and, with the sponsor's help, he was given access to areas of the building that are usually off limits to the public. Here is just a glimpse at some of the images he captured. If you love this particular project, you should also see David's series on the New York Public Library.

All images © David Katzenstein