Grand Central Terminal is one of the most dramatic and enduring symbols of New York City. An architectural and technological marvel built almost a century ago, it has triumphed over the car and jet-age, corporate greed, the wrecking ball, even the city’s indifference.

Facing different challenges today, this magnificent train station is reinventing itself for the future. Inside Grand Central takes you into the heart of one of America’s most impressive mega-structures and reveals all, from the terminal’s secret operations 10 stories underground, to its public facade that towers over Park Avenue.

The building that rises today on the corner of Park Avenue and 42nd Street was preceded by another building that served as a rail terminal: the Grand Central Depot. The Depot, the biggest indoor space in America in 1871, was the brainchild of industrialist Cornelius “Commodore” Vanderbilt, founder of the New York Central Railroad. The Commodore, so called because of his shipping interests, was one of the original railroad “robber barons.” Through force and manipulation, he was the first to successfully link New York City and Chicago with one rail line. But the Depot was the victim of its own success: after 30 years of operation it was too crowded and unsafe for operation.

A horrific railroad accident in 1902 convinced the New York Central to electrify the rails, exchanging noxious, loud, and dangerous steam locomotives for sleek electric trains. The genius behind Grand Central Terminal’s innovations, including its electrification, internal ramp system, and circumferential drives, was New York Central’s chief engineer William Wilgus. With the decision to electrify Grand Central, rail yard operations were moved underground and construction began on the new breathtaking Beaux-Arts building, completed in 1913. Grand Central was now the epicenter of midtown Manhattan with a “Grand Central District” of office buildings and hotels built around it, connected by underground tunnels. In the heyday of rail travel, from the turn of the century until WWII, Grand Central experienced peak demand. However, demand for the terminal soon faltered.

Threat and survival would become themes of Grand Central in the coming decades. The onslaught of the car and jet as alternative forms of travel in the 1950s, the desire to erect a skyscraper over the terminal in the 1960s, the fiscal neglect of cash-strapped New York City in the 1970s, and the growth of the terminal’s homeless population in the 1980s each could have destroyed Grand Central, but it emerged stronger after each trial. Grand Central now confronts an issue facing public transit systems worldwide: terrorism. Inside Grand Central follows the MTA’s Emergency Service Unit for a detailed look at the terminal’s disaster planning and fortifications.

Today, the terminal is as vibrant and alive as ever, and we see firsthand how this mega-structure operates hour-by-hour in all its glory. We travel 10 stories below to room “M-42” where the electricity that powers the terminal and its rails is generated. We explore the service plant six stories below ground and follow 56 miles (90 kilometers) of steam pipes that power the building’s heating and cooling systems. We climb to the top of the roof where a sculpture called “Transportation” depicts the figures of Hercules, Minerva, and Mercury gazing over Park Avenue. We go behind the ticket counters and information booths, and ride the rails on trains that leave its platforms.

Part of Grand Central’s success is its ability to adapt with the times, and one way the terminal is anticipating the future is with the “East Side Access” project. This new underground development will bring in an additional 80,000 commuters from Long Island. Our detailed graphics show exactly how the new rail extension, and new terminal expansion, will look in 2012.

Inside Grand Central is a stunning panorama of one of the most important rail facilities in the world and how it evolved into the lasting mega-structure it is today.