Subway construction in New York had not hit its peak when Scientific American published its issue about the city. In 1908, the only real subway was the original Interborough (IRT), zig-zagging north up Manhattan. Shortly afterward, the city signed “Dual Contracts” with the IRT and the Brooklyn Rapid Transit Company, later renamed Brooklyn-Manhattan Transit. That led to the creation of about two-thirds of the current system in the 1910s and 20s.
The most prominent tunnel construction at the time of publication and shortly before was the Hudson & Manhattan Railroad, now the PATH train. This was (effectively) a subway that connected the railroad terminals in Jersey City and Hoboken to Manhattan, which mean that its critical parts were tunnels under the Hudson River and some north-south underground trackage along the New Jersey waterfront. The picture above is a somewhat fanciful but not entirely incorrect view of the intersection between the river tunnel and the north-south tracks. This was a difficult piece of design and construction, entirely within alluvial mud.
The second page has three pictures that require a bit of explanation. The one on the lower right is the clearest: a section showing the two Hudson River tubes then being built for Pennsylvania Station, which opened in the fall of 1910. The one on the top is not directly connected to tunneling: it’s the interior of IRT’s powerhouse, at 59th Street and the Hudson, where barges could supply coal directly. The powerhouse still stands, long since converted to general electric-power generation, and with a fantastic facade designed by McKim Mead and White.
The section on the lower left, along 33rd Street at Herald Square, is a minor masterpiece, showing the complexity of the rapidly-growing transportation network. From top to bottom we have the Sixth Avenue elevated, the Broadway streetcars, the Hudson & Manhattan uptown branch (in construction), the Broadway subway (planned as part of the discussion that led to the Dual Contracts), and the Penn Station tunnel leading east to the East River and Queens. Five layers of rail crossing along three axes.