Over the past few years, talk has ebbed and flowed around digging new railroad tunnels under the Hudson River between New Jersey and New York. The existing tunnels were damaged by flooding from Hurricane Sandy, there's ongoing wear and tear from the passage of dozens of passenger trains each day, and the need exists to upgrade the tunnels’ electrical power and signaling system, among other tasks (see here). Estimates are that the planning and building process for the so-called Gateway Program could take 10-20 years at a cost of $10-$20 billion dollars.

(Editor's Note: On November 12, federal officials agreed to fund at least half of the Gateway tunnel project after a deal was reached to create a development corporation to oversee the project.)

Indeed, the Gateway Program is still in the planning and design phase and a reliable program cost estimate has not yet been developed, according to Amtrak, which operates the U.S. passenger rail service. Amtrak has directed more than $300 million, mostly from federal sources, to the Gateway Program since 2012. This includes some $74 million for planning and pre-construction work and $235 million to fund the Hudson Yards concrete casing project from federal Sandy Resiliency funding under the Disaster Relief Appropriations Act of 2013.

The tunnel plan through the Hudson River riverbed was a straight line from above, but in profile descended from the shore and then rose at the other shore with steep grades at each end.The tunnel plan through the Hudson River riverbed was a straight line from above, but in profile descended from the shore and then rose at the other shore with steep grades at each end.What may be more astonishing is that the existing tunnels are more than 100 years old, and represent unappreciated marvels of civil engineering and human labor. Despite many challenges, they were completed in less than five years at a cost of about $50 million (in 1900 dollars), far less than the projected cost of the new tunnels even after accounting for inflation.

In a sad twist, the tunnel's terminus at the Pennsylvania Railroad Station in midtown Manhattan, built concurrently as an "eternal temple" to railroads in general and the Pennsylvania Railroad in particular, was torn down in 1963 as obsolete after 50 years of service. A terminal that many daily commuters consider to be uninspiring currently occupies the space.

A Watery Barrier

But all that was still decades in the future when Alexander J Cassatt, president of the Pennsylvania Railroad, sought a solution in the early 20th century to a problem that been an irritant to efficient railroad operation: the connection between mainland New Jersey and Manhattan Island, hub of the growing metropolis of New York City.

In the late 1800s, New York City was already the business center of the United States. However, it was cut off from the mainland U.S. by the mile-wide Hudson River. As a result, train travel was forced to stop in New Jersey, cargo had to be loaded onto barges and passengers had to board ferries, which departed terminals every 10 minutes or so.

Despite the inconvenience, there did not seem to be a better alternative. A bridge seemed obvious, but there were issues with access roads, ownership rights with other railroads and local politics. A tunnel seemed technically impossible, due the muddy silt of the riverbed, which could not properly support a tunnel bored through it, or so everyone thought. Even if a tunnel could be built, the smoky exhaust of the coal-powered steam engines would likely choke the locomotive’s boilers, not to mention passengers and crew.

Electrification Offers a Solution

Everything changed when Cassatt, with broad and deep experience in railroad operations and with a civil engineering degree from Rensselaer Polytechnic Institute, went to Europe and saw the Gare de Quia d'Orsay train station in Paris. He was awed by 45-ton electric locomotives that pulled 300-ton passenger trains through tunnels along the Seine. Here was the answer to the tunnel-exhaust problem and a vision: electric-powered trains going directly into a tunnel under the Hudson River and arriving at a train station "built for the ages" in Manhattan.

The dry side of the shield boring machine. It had nine doors through which workers could shovel away the riverbed by hand.The dry side of the shield boring machine. It had nine doors through which workers could shovel away the riverbed by hand.Of course, vision is one thing, implementation is another. Underwater (or submarine) tunnels were difficult to build and often unsuccessful, with many begun and later abandoned. Cassatt hired the foremost tunnel builder in the world, Charles Jacobs, along with a leading civil-engineering manager, Samuel Rea. Together they worked out a plan. Cassatt also hired the leading architectural firm of the day, McKim, Mead & White, to design an opulent station that would represent everything the railroads—and the Pennsylvania Railroad—hoped to symbolize.

Cassatt also decided that his railroad would plan, manage and execute the plan all by itself, including funding its entire $50-million budget.

Defying the Skeptics

Critics scoffed that the tunnel could not be built, and they had good reason to say so. Another tunnel attempt nearby had been abandoned a decade earlier, mostly due to problems with the unstable silt riverbed. Others said that there was no tunnel structure that could withstand hundreds of heavy trains passing through every day. They said the tunnel would distort and fracture from the vibration as they flexed on the soft river bottom, or that they simply would bend and split under the loads.

None of this stopped Cassatt and his team, however. On June 24, 1903, men and machines began digging two holes 30 feet across and 55 feet deep, one on the New Jersey side and one on the Manhattan side. Instead of a single larger-diameter tunnel, there would two 23-foot-diameter parallel tunnels; both would be started from opposite sides of the river and meet in the middle.

Use of dual tunnels was selected in part to reduce the diameter needed for each tunnel but also to prevent tunnel blockages or pileups if a train were to derail. Further, if one tunnel was dedicated solely for eastbound use and the other for westbound, the only way there could be a collision was if a train was stopped and the train behind it proceeded unawares. Of course, this was preventable with basic signaling.

Digging the tunnels was done by mostly by hand. Thousands of workers called “sandhogs” worked 24/7 under the riverbed with pneumatic drills and hand shovels to scoop away the debris. The debris was put into carts which were pushed to a down-hole, where the carts were raised to the surface and taken away by locomotives that pulled small hopper cars that followed specially laid tracks.

Digging the Tunnels

The push forward was slow and progress was anything but steady. The tunnel-boring machine at the face of the digging on each side was the "shield" (pictured above) a 200-ton, 17-foot long, 23-foot-diameter mechanical mole with nine doors. One door at a time would be opened and a laborer would shovel some of the exposed silt and muck into the open work area. The door then would be closed and another one opened to repeat the extraction and shoveling process. When all the doors had been opened, emptied and closed again, a hydraulic ram would push the face of the shield forward into the cleaned-out area just ahead.

The DD1 locomotive was designed for the tunnels. The bulk of its mass was a huge electric traction motor.The DD1 locomotive was designed for the tunnels. The bulk of its mass was a huge electric traction motor.When the push had advanced a few feet, workers would bolt a dozen cast-iron segments (each 2 ½ feet wide) to form a ring at the area at the back-end of the machine. In this manner another section of the tube was completed. Assembling and bolting the ring segments took six hours at the start of the construction project; as the process was improved, it could be done in as little as 90 minutes. Progress was measured at between two and three feet per day, but there were many instances when no work could be done due to technical problems.

To keep the river and silt from coming in when each door was opened, huge compressors on the surface pumped air at about 20-30 psi into the tunnel. Decompression sickness, also known as the "bends," was a constant danger despite special acclimation zones and air locks built into the tunnel entrances. There also were many blowouts where the compressed air punched through a weak spot in the muck. Thhese blowouts resulted in huge jets of pressurized air shooting to the surface, sometimes taking workers with them.

Tunnel Alignment

Tunnel alignment was a major issue and there were no laser-based surveying instruments, GPS locators, computers for extensive calculations that today are standard tools of surveying metrology. Instead, a team of more than a dozen surveyors was responsible for making measurements of each tunnel as it advanced, and then relaying their calculations to the surface.

A 60-foot-high tower was built on the New Jersey side as a zero-point of reference for the complex surveying triangulation needed to assess both the vertical and horizontal alignment as each section was added. As the survey team determined deviations via measurements, they would relay instructions to the tunnel crew to adjust the trim of each newly bolted on ring section with the use of shims. The entire measurement and adjustment process could take hours to complete for each ring.

Despite many technical challenges and setbacks, the two sides of the north (upstream) tunnel met and punched through on Sept. 11, 1906 at a depth of 97 feet below high tide. Then came the moment of truth: how close to perfect alignment were the tubes where they met? A test sighting was performed through a small hole at the meeting point, and the misalignment was found to be between 1/16 and 1/8 inch, an amazing accomplishment in surveying. Based on this, the workers completed digging and the north tunnel core was complete.

The north tunnel was completed a year ahead of schedule; the south tunnel was connected a month later with similar accuracy. There was still much electrical work to be done and track had to be laid, but the basic tunnel was ready. On Sept. 12, 1906, management of the railroad and its engineering team did what many thought would have been impossible: they walked under the Hudson River from New Jersey to Manhattan. The public acclaim was similar to the moon landings 60 years later. Sadly, railroad President Alexander Cassatt suffered a fatal heart attack on Dec. 28, 1906.

The waiting room at Pennsylvania Station. Trains were reached on 22 tracks below this room.The waiting room at Pennsylvania Station. Trains were reached on 22 tracks below this room.Even after successful completion, a stream of data from the surveyors showed that both the north and south tunnels were moving up and down every day—and this was before any trains passed through. The problem was studied for months, with no clear idea why this was happening or if this motion would result in one or both tunnels cracking.

Eventually, it was decided that the tidal motion of the Hudson River exerted pressure on the tunnel tubes and caused the motion. The good news was that the cause had been found; the bad news was that no one knew if this repetitive action would be a problem. We know the answer today: the tunnels are still intact more than 100 years later.

Custom -designed DD1 locomotives from Westinghouse and Baldwin Locomotive Works were used to move trains from New Jersey to the Pennsylvania Station in midtown Manhattan. These DC-powered units used a third rail and could start 850-ton trains up the steep 1.94% grade in the westbound tunnel. The double-ended locos (no need to turn them around) were more powerful than available steam-powered engines, could reach 80 mph and had a good reliability record (averaging 1 minute of failure for every 11,500 miles traveled).

Huge and Opulent

Only a few words are needed to describe Pennsylvania Station itself: huge and opulent; photos can only hint at it (see Reference 6). Although intended for the masses, it was a palace, a "cathedral of the industrial age" and "a monument for the ages." After six years of construction, Pennsylvania Station opened to the public on Nov. 27, 1910.

Although there was a budget and schedule, many design decision were made based on esthetics more than cost. There was a short-lived plan to place a hotel on top of the station for revenue. This plan was discarded primarily because it would ruin the sheer grandeur and imposing presence of the station (and also would require removing two of the tracks to make room for supports).

From an engineering perspective, the major design challenges were below the huge main floor with its waiting room. The station serviced 22 main tracks, with extra tracks providing the equivalent of full train-yard operations and service. To build this underground railyard, a pit between 60 and 100 feet deep was dug across the entire site and extended many blocks west to the tunnel mouth at the Hudson River. While this pit was dug (again, largely by hand and with pneumatic tools), the streets along its edges were shored up and the elevated train lines which ran above them were buttressed so there would be no service or access interruptions.

The End of Pennsylvania Station

When the Hudson River tunnels and Pennsylvania Station were completed, no one foresaw that by the 1940s the primary mode of personal transportation would increasingly be cars, with airplanes for longer distances. Freight moved on trucks with their greater logistics flexibility and on improved roads.

Pennsylvania Station brought travelers to the heart of Manhattan via tunnels that today are more than 100 years old.Pennsylvania Station brought travelers to the heart of Manhattan via tunnels that today are more than 100 years old.In 1946, the Pennsylvania Railroad for the first time lost money. The situation continued to decay, and by 1960 the railroad was approaching bankruptcy. Management decided to demolish Pennsylvania Station and sell the air rights above it. Where the railroad station once stood, Madison Square Garden – a venue for sports and concerts – now stands. The rail station that now operates from the location is functional if uninspired. The original Pennsylvania Station's demolition began on Oct. 28, 1963, just 50 years after it opened.

Completion of the twin-tunnel and passenger rail station project ahead of schedule (less than five years from concept to completion) and on budget, and without many of the tools or techniques we now have, is a testament to the vision, determination, and energy of the men who made it happen despite challenges. Indeed, engineers designed and completed both the tunnels and the station in less time than has been spent debating how to maintain this still-vital rail connection between Manhattan Island and mainland United States.


  1. Jill Jones, "Conquering Gotham—A Gilded Age Epic: The Construction of Penn Station and Its Tunnels," Viking, 2007
  2. "The Rise and Fall of Penn Station," American Experience PBS Video, 2014
  3. Lorraine Diehl, "The Late, Great Pennsylvania Station," Four Walls Eight Windows, 1996
  4. http://www.nypap.org/content/pennsylvania-station
  5. http://mashable.com/2015/07/20/original-penn-station/#oIo3aVgIZkqG
  6. http://www.citylab.com/design/2013/10/10-gorgeous-nostalgic-photos-new-yorks-old-penn-station/7384/
  7. https://insights.globalspec.com/article/1455/focus-on-speed-safety-drove-multiple-innovations-at-sea
  8. https://insights.globalspec.com/article/1648/transatlantic-telegraph-cable-engineering-innovations-still-used-today