Opening a New Link of New York's Vast Subway System (1918)

Scientific American · July 27th, 1918

Difficulties of Converting a Tunnel Into a Bridge Without Interrupting Traffic

The opening of the Lexington Avenue branch of the New York Subway line last week marked the culmination of one of the most difficult engineering problems ever carried out in New York City. At present, only the local tracks are in service from Forty-Second Street to 167th Street on the Jerome Avenue line. In a week or two, it is expected that the express service will also be installed, and then trains running up the present Fourth Avenue line, instead of crossing the city at Forty-second Street and proceeding up the Broadway line, will travel up Lexington Avenue. At the same time, trains coming down Broadway above Forty-Second Street will turn at Times Square into Seventh Avenue and proceed downtown on the west side of the city. Connection between the East and West Side lines will be maintained by a shuttle service across Forty-second Street. This will form the horizontal bar of the so-called "H" system.

The intersection of Forty-second Street and Park Avenue will thus become one of the most important traffic centers of the city; for here we have the junction of the Lexington Avenue line with the Forty-second Street shuttle, and on a still lower level there is the Queensborough tunnel. While overhead, there is a branch of the Third Avenue Elevated Railway, with another flow of traffic furnished by the New York Central and the New York, New Haven and Hartford Railroads at the Grand Central Terminal.

In the Scientific American of July 17th, 1915, we described in detail the engineering work involved in diverting the existing subway from its present course to Lexington Avenue. The accompanying drawing gives a general idea of the arrangement of tracks and platforms at this important junction. The new subway tracks branch off from the old in the region of Thirty-ninth Street, where they run down grade until they have acquired sufficient headroom to pass under the old subway tracks, and branch off toward the east, except for the northbound local, of course, which turns off without passing under the existing tunnel. Here a new station has been built, which is locally known as the "Diagonal Station" because it runs diagonally from Forty-first Street and Fourth Avenue to Lexington Avenue north of Forty-second Street. A large part of the station is built in an open excavation where formerly stood the Grand Union Hotel, and this open plot provided a vantage point from which the new tunnels were pushed under the existing subway line and connected with them. It will be recalled that above the subway on Park Avenue there is another tunnel through which run the street cars. So that, altogether the rock at this point is fairly riddled with tunnels- in fact, it has earned for itself the name of "the honeycomb."

Junction of three subway lines at 42d Street.

As shown in the drawing, the northbound tracks of the existing subway run under the east side of Park Avenue and are separated by a core of rock from the tunnel in which the southbound local and express tracks run. A tunnel for the new north and southbound express tracks has been cut through this core of rock between the tracks of the existing subway. It was an exceedingly difficult task to cut away the core of rock between the north and south bound tunnels of the old subway without precipitating a slide upon the work. In this region the rock is very poor indeed; it is full of slanting seams. Some of the rock is very hard, but much of it is soft with large pockets of disintegrated rock. This is so soft that on exposure to the air for a few hours, it may be broken off with the fingers and pulverized in the hand. No reliance whatever could be placed upon the stability of the rock to support the roof while the operations were proceeding. The problem was to cut away the inner half of the arch of each existing tunnel and remove the core of rock between them without interfering with the heavy traffic at this point. When it is realized that during the rush hours there is a train passing on one of the four tracks every thirty seconds and that in the neighborhood of a million passengers go over this line each day, the hazardous nature of the work will be appreciated. The slightest mishap might have precipitated an appalling catastrophe. The tunnels were first lined with a sheathing of timbers to prevent materials from dropping down upon the cars. Then the core of rock was removed by the nibbling process so that there was little unsupported rock at any one moment.

Left: One of the huge girders carrying the old subway and supporting the roof of the new. Right: Three drifts extending from the Grand Union plot under the old subway. [Sorry about the way these photos are cropped; it is the way they were originally published.--Webmaster]

One of the most interesting bits of work was that of passing the new subway under the old. Here the existing subway had to be changed from a tunnel to a bridge. It was necessary to support the old subway upon a system of girders which would also carry the roof of the new subway. Access to the site was had through drifts that were run from the Grand Union plot under the existing subway. Pits were sunk from the floor of the old subway, and in some cases stoped up from the drift below, into which the columns were set up. The girders were to run parallel to the tracks of the existing subway and hence diagonally across the new tracks. Alongside the tracks trenches were cut in the floor and rock to receive the big girders. The girders varied in size from 26 to 48 feet in length. They were 72 inches deep, back to back of flange angles, and weighed from 7.5 to 35 tons. So heavy were they that it was deemed inadvisable to attempt to lower them into place from the street. Had this been done it would have been necessary to interrupt street traffic, and cut upon the street decking and lowering the enormous girders into place. Instead of this, the girders were brought into position from beneath. Accordingly, a hole was cut from the drift of the new tunnel up to the level of the existing subway. This was cut on a slant so that it formed a ramp up which the girders could be skidded. After being brought up to the level of the existing subway the sheathing of the subway was cut out to permit of skidding the girder upon the express tracks. This work was done after one o'clock in the morning, when, for a period of five hours, express service is discontinued. Then, with the aid of a motorcar, the girder was hauled to the proper position and turned over on edge into the trench where it was properly positioned upon the columns that were to support it. After the girders were in place the new floor of the subway was built in, piece by piece; and in order to prevent the tracks from sagging at any point they were bound together by means of "H" girders whose flanges were fitted over the ends of the ties at the section where the work was being done. These "H" girders were fastened together by means of tie bolts, and they formed the track into a rigid bridge structure.

The placing of the steel to support the old subway tracks called for some very careful surveying. Traverses had to be run through the drifts and around the various tunnel openings to locate the positions for the columns. Despite the complexity of the work everything was most accurately calculated so that when it came to placing the final piece of steel it proved to fit perfectly and was driven home with a few taps of a heavy beam. No shimming and no reaihing had to be done on the work. This is a tribute to the careful and painstaking work of the engineers in charge.

Left: A line of girders in the old subway. Note that the sheathing at the right has been removed to admit one of the big girders. Right: >Where the new northbound local meets the old. The grade of the existing line must be lowered so as to run into the Diagonal Station.

By glancing at the accompanying drawing it will be noticed that the northbound local track does not pass under the existing subway but joins it just below Forty-first Street. The Diagonal Station is placed at a much lower grade than the present subway station, and were the northbound local to rise from the station to the level of the existing subway at the point the two intersect the grade would be too steep for its intended use.

The building line at Forty-first Street is close to the existing subway so that the new track cannot run off to the side and the only way in which connection can be made with the north bound local is to lower the grade of the existing tracks at this point. One of our photographs very clearly shows the difference in level at this junction.

The new local tracks pass under the old northbound local for a considerable distance, and when the Fourth or Park Avenue line is finally connected with the Lexington Avenue subway, the existing northbound local track will have to be cut away and connection can then be made with the new track at a point near Fortieth Street.

In our drawing the various levels are indicated by differences in shading. There is a broad mezzanine platform extending over the Diagonal Station and connected with the present subway station by means of a ramp. In this mezzanine platform there are a number of large elevators which run down to the Queensborough tunnel, formerly known as the Belmont tunnel, and prior to that as the Steinway tunnel.

The present subway platform will be extended so to bring the shuttle trains nearer the Diagonal Station. A large part of the Lexington Avenue subway is double-decked. The express tracks being on the lower level and the local tracks on the upper level, where they are more readily accessible to the street. A very interesting development is to be found at the 125th Street station. Here the subway performs a corkscrew twist which is very complex but which has a very good reason for its existence. As the subway is in two levels it has been planned to have downtown trains on the lower level and uptown trains on the upper level, the purpose of this being that, as most of the traffic is southbound in the morning, there will be no hardship in going down two flights of stairs to reach the lower level. In the evening, when the rush is northbound, tired passengers will be much relieved to find themselves on the upper level with only a single flight up to the street; and thus, while both express and local northbound trains are on the upper level here, we find further downtown that the expresses have been separated from the locals, with the express trains on the lower level and the locals on the upper level.

In the region above 125th Street there is another complication. Trains, after passing under the Harlem River, branch toward the west and the east, one line running under Jerome Avenue and the other under Westchester Avenue. Trains coming from these two branches down to 125th Street remain upon their own side of the track- those from the one branch on the west side and the others on the east side, whether they are locals or expresses. After passing the station they are routed on the express or local tracks. Northbound trains, on the other hand, are switched over after passing the 125th Street station according to the branch that they are to take. In order to avoid switching upon coming into a station and to allow for it after the station has been passed, a very complicated arrangement of tunnels is provided.

In a general survey of the new line one cannot but be impressed with the intelligence and foresight shown in planning this piece of New York's great subway system. At the same time the work stands out as one of the most notable undertakings in a city renowned for its big and daring engineering achievements.