There are a variety of types of tunnels that can be built. Primarily they are the cut and fill and the true tunnel. In the cut and fill method, which is used because it is much cheaper to construct, a trench is dug, the tunnel walls and ceiling are constructed of masonry or other materials, and then the earth is returned to the trench to "bury" the tunnel. The true tunnel is constructed by literally digging from one end to the other (or, more often, digging from both ends towards the middle). Some true tunnels are dug in dirt, and others through stone. The advantage of stone, many times, is the lack of need for a supporting structure, but the disadvantage, of course, is that the tunnel must be cut from the very strong stone, which makes it much more difficult and time consuming, not to mention the cost.
The oldest true tunnel in NYC for rail traffic, or any kind of traffic, is the Mount Prospect Tunnel in Manhattan, opened in 1837. It now forms the center two tracks of Metro North from 92nd St. to 94th St. under Park Ave. The tunnel north and south of it, and the one-track tunnels on each side of it, were added in 1873-1875, but the Mount Prospect Tunnel was left in place and became part of the larger “Fourth Avenue Improvement”
The following illustration that appeared in the November 14, 1874 issue of the Scientific American shows the ground elevation and other pertinent information on the underground rail line from 42nd St to the Harlem River which was completed in 1875. Of course, Fourth Avenue, became Park Ave.
In the center frame I have enlarged below you can clearly see that the section between 92nd and 94th Streets is labelled "Rock Tunnel."
Interestingly, the area that the Mount Prospect Tunnel is located is now called the Carnegie Hill area of the upper East side of Manhattan. That name, Carnegie Hill, became the new name for Mount Prospect after Andrew Carnegie purchased land (1897) and built his residence (1903) at 91st Street and 5th Ave., a few blocks away from the tunnel.
The Atlantic Avenue Tunnel (also referred to as the Cobble Hill Tunnel) in Brooklyn was built in 1844-45 and is touted as the oldest Railway Tunnel in the world, it has been cited that the Mount Prospect Tunnel, while having been built earlier, was originally built for street cars and only converted to be a railway tunnel in the mid 1870s.
|Cobble Hill Tunnel|
There is a belief that a true tunnel was constructed for the Croton Aqueduct in upper Manhattan which opened in 1842, but that has not been documented as of this writing. Clearly, that tunnel, if it exists, was not for traffic at all, but rather a viaduct for water and appears to be constructed around five years after the Mount Pleasant Tunnel was completed.