One of the most interesting sections of the work is that which approaches and passes under the Harlem River, carrying the two tracks of the East Side line.  The War Department required a minimum depth of 20 feet in the river at low tide, which fixed the elevation of the roof of the submerged part of the tunnel.  This part of the line, 641 feet long, consists of twin single-track cast-iron cylinders 16 feet in diameter enveloped in a large mass of concrete and lined with the same material.  The approach on either side is a double-track concrete arched structure.  The total length of the section is 1,500 feet.

The methods of construction employed were novel in subaqueous tunneling and are partly shown on photographs on pages 62 and 63.  The bed of the Harlem River at the point of tunneling consists of mud, silt, and sand, much of which was so nearly in a fluid condition that it was removed by means of a jet.  The maximum depth of excavation was about 50 feet.  Instead of employing the usual method of a shield and compressed air at high pressure, a much speedier device was contrived.

The river crossing has been built in two sections.  The west section was first built, the War Department having forbidden the closing of more than half the river at one time.  A trench was dredged over the line of the tunnel about 50 feet wide and 39 feet below low water.  This depth was about 10 feet above the sub-grade of the tunnel.  Three rows of piles were next driven on each side of the trench from the west bank to the middle of the river and on them working platforms were built, forming two wharves 38 feet apart in the clear.  Piles were then driven over the area to be covered by the subway, 6 feet 4 inches apart laterally and 8 feet longitudinally.  They were cut off about 11 feet above the center line of each tube and capped with timbers 12 inches square.  A thoroughly-trussed framework was then floated over the piles and sunk on them.  The trusses were spaced so as to come between each transverse row of piles and were connected by eight longitudinal sticks or stringers, two at the top and two at the bottom on each side.  The four at each side were just far enough apart to allow a special tongue and grooved 12-inch sheet piling to be driven between them.  This sheathing was driven to a depth of 10 to 15 feet below the bottom of the finished tunnel.

A well-calked roof of three courses of 12-inch timbers, separated by 2-inch plank, was then floated over the piles and sunk.  It had three timber shafts 7 x 17 feet in plan, and when it was in place and covered with earth it formed the top of a caisson with the sheet piling on the sides and ends, the latter being driven after the roof was in place.  The excavation below this caisson was made under air pressure, part of the material being blown out by water jets and the remainder removed through the airlocks in the shafts.  When the excavation was completed, the piles were temporarily braced and the concrete and cast-iron lining put in place, the piles being cut off as the concrete bed was laid up to them.