To be fair, Grand Central Terminal occupies far more than an ordinary city block, but that's besides the point: it holds up as a conceptual unit of urban space, much like the city block, and the normal course of pedestrian traffic passes through it without having to cross a street, another characteristic generally considered to define a city block.
As you approach from the south, the first thing you notice is how majestic the Terminal building is. It is an artifact of the wealth from New York's Gilded Age, conceptualized in 1902 and completed in 1913. But let's leave aside beauty for our current purposes, and just keep in mind that it exists. The first piece of ingenuity you notice is that while the Terminal blocks the main lanes of Park Avenue from continuing north past E. 42nd St, a ramp rises from the middle of Park Ave and E. 40th St, carrying one lane of traffic in each direction above the pedestrians and the ground level and then around the Terminal, northbound traffic to the east and southbound traffic to the west. To the east, a second lane branches off, for passenger pick up and drop off from the Grand Hyatt, because of course there is a 26-story luxury hotel attached to Grand Central Terminal in such a way as to be completely joined at the ground level (interesting fact: this was Donald Trump's first major development project).
The best part of the viaduct however is on the north side, where it carves through the Helmsley building, depositing traffic right back onto Park Avenue at 46th St. This is the greatest example of vertical layering: you have a road that not only goes through a building in two separate places (to say nothing of the dozens of train tracks beneath the ground), but that makes 90 degree turns while doing so. It all seems to be so well planned, but in fact the viaduct was not proposed until three years after GCT had opened, and when it was built in 1919, it originally went only around the west side of the building, causing terrible traffic jams at its termination at 45th and Vanderbilt. It wasn't until 1928-29 that the Helmsley building with the current viaduct ramps passing through it to 46th St was built.
The final piece of the genius of this block, at least at the macro level, is the MetLife Building. A lot has been said about how much New Yorkers hate it, although I suspect that might have been more true of an earlier generation. To me it seems to float above the Terminal, even though in reality it sits on a lot immediately to its north. The reason that New Yorkers of an earlier generation hated it so much is the same reason that I love it: it blocks Park Avenue from either side, drawing the eyes down to Grand Central itself. What do I care if it is over 50 stories high? So is everything else in midtown these days it would seem.
These three aboveground layers - terminal, viaduct, skyscrapers; all built at different times, yet fitting together so ingeniously, as if they had always been meant to be - is what is so great about this city. And that is again, to say nothing of the 44 platforms which make Grand Central Terminal the largest train station in the world - well into my second decade as a New Yorker underground trains are something I take for granted by now. Although I do have to say that adding a special track for presidential visitors is kind of a neat trick ...