Well-Planned Schedules Speed Subway Track Construction (1931)
Electric Railway Journal · Vol. 75, No. 7 · July, 1931 · pp 347-349.
A finished track ready for service.
Well-Planned Schedules Speed Subway Track Construction.
By Robert H. Jacobs, Division Engineer, Board of Transportation, New York City
Standards of track construction for the new subway lines in New York City represent a normal development based largely on experience of the companies operating existing rapid transit lines. As in the construction of the subway structure, the specifications for all track construction and materials are prepared by engineers of the Board of Transportation. The work itself, however, is done by local contractors who have been awarded contracts for definite sections of the line, depending upon the continuous length of completed structure. In the past, contracts have varied from lengths of 5 to 20 miles of continuous track. Engineers of the Board of Transportation with their inspectors are in the field during the progress of the work to insure that it is done in strict conformity with the specifications, and to give all necessary lines and grades for the work. All track material, except concrete and a few minor items, is purchased by the Board of Transportation and furnished to the contractor.
The most important change from former track construction practice is the adoption of the concreted type of track throughout, except for special work, short sections of track between special work, lay-up tracks and yard tracks. The rails are independently supported on short creosoted ties of long-leaf heart-wood pine, 2 ft. 6 in. long, spaced, 22-1/2 in. apart with an 18-in. spacing at the rail joints. Longer ties are used for the third-rail supports. Concrete is poured to the top of the tie with a slight slope toward the center of the track. A trough, 2 ft. 5 in. wide and from 7-1/2 in. to 10-1/2 in. deep at the center, is formed between the ties, with drain boxes at intervals as required to meet local conditions. Screw spikes are used with the tie plates having bosses to support the heads of the spikes so as to leave a space of
The subway structure is built to conform to theoretical dimensions within the limits of practical construction practice. The small operating clearances planned require that a detailed survey of the structure be made before installing the tracks, and in some cases alignments and grades must be adjusted to assure proper clearances. Line and grade points are established on the subway walls at 20-ft. intervals, and the location of insulated joints, switch machines, automatic stops, and conduits for feeders, negative returns and other cables are also marked. This work is well advanced before the actual track construction.
Methods for Getting Materials Into Subway. Rails, special work and fittings are hauled by trucks of the track contractor from convenient freight stations to the work. Ties are delivered along the site of the work at points convenient for getting them into the subway. Selection of such places is an important factor governing the progress of the work as well as the cost to the contractor. The rails, special work and most of the fittings are usually introduced into the subway through suitable station entrances. A portable power hoist unloads the rails from the trucks after which they are skidded down temporary ramps to the station platform. The rails are then loaded with a block and fall on flat cars driven by a gasoline engine. About 30 rails, with angle bars and track bolts, can be loaded on each flat car. A derrick on the flat car places the rails continuously along the trackways of the subway where they are immediately bolted and gaged loosely, allowing the car to proceed and to lay more rail.
After the rails for a designated section have been placed and bolted, ties and track fittings are distributed along the line at convenient points. The ties and tie plates are set in place by raising the rails with jacks. After the track is raised to the approximate line and grade, precast concrete pedestal blocks, spaced 8 ft. apart, are placed under the base of the rail. These pedestal blocks are made to the same specifications as the concrete in the track and become a part of the track structure. The tops of the pedestal blocks are set about 2 in. below the base of the rail for the insertion of wedges to make the final adjustment. The rails rest on these blocks so that the bottom of the ties hang about 7-1/2 in. above the invert. In general, four line and four gage braces are used for each rail length on tangent tracks, and five on curves to hold the track in place. These braces are cut short to permit final adjustment of the line and grade by wedges. After the track is checked for line, gage and surface, concrete is poured. The only forms necessary are those placed along the ends of the ties to construct the trough. Construction of the concrete track is carried on and completed in sections of 1 mile.
The concrete mixer, having a capacity of 1 cu.yd. is located on the street surface at about the center of the section to be concreted. Concrete is conveyed by chute to side-dump buckets in the subway through a 10-in. galvanized iron pipe chute. The chute is carried from the street surface through a duct manhole or a ventilating chamber to the buckets. The pouring of the concrete is begun at the two extreme ends of the section simultaneously, progressing toward the center. Six buckets, each having a capacity of about 2 yd., are driven by a gasoline locomotive under the chute to receive the concrete. Three buckets are placed on each side of the locomotive, one set of buckets being filled and driven to the end of the section and dumped, while the other set is left under the chute to be filled with the concrete. The engine then returns to the chute with the empty buckets, hauls the others, which are filled by this time, in the opposite direction. This operation is repeated, finally closing in the works at the chute location. For greater speed two locomotives are sometimes used.
Batches of Coarse Aggregate Hauled from Yards. Rigid inspection of the work and frequent checking of the track for line and grade are necessary, as the success of this type of track is largely dependent on its accuracy and the thoroughness with which ties are embedded in the concrete. The proportion of aggregates, the control of water and the thorough mixing of the concrete follow the accepted practice for this class of work. The aggregate is delivered to the mixer in compartment trucks, each compartment carrying the correct proportion of sand and crushed stone for a batch. The aggregate is unloaded directly from the truck to the mixer. Concrete is mixed in the proportion of 1 : 2 : 3.85, 3/4-in. crushed stone being used for coarse aggregate.
The concrete is poured for the full depth of the track construction in one operation and thoroughly spaded or tamped under the tie. Depressions in the surface of the concrete are provided for the placing of rail anchors. After the concrete is poured and thoroughly tamped, the surface is finished by troweling. Care is exercised in troweling the surface of the trough in order not to honeycomb the concrete around the ties. Concrete is poured at the rate of 800 to 900 ft. of track per working day, exclusive of the time taken in changing the location of the mixture. About 1/3 cu.yd. of concrete is poured to a running foot of track.
Prior to pouring the concrete, a full-sized clearance template is run over the track to insure that proper car clearances have been provided. Concreting is followed up promptly after the track has been finally checked by the track inspector with special gages and templates and necessary adjustments are made to bring the track to accurate line, gage and grade, corresponding with the marks on the walls of the subway. No construction trains are allowed to run over the fresh concrete until after a period of at least 36 hours.
As a measure of economy and in the interest of faster work, the laying and bonding of the contact rail, installation of the insulated joints and of conduits in the track ways are included in the track contract. The contractors' equipment for handling the track material is also suitable for handling the contact rail. In this way, the work can be carried on in connection with the construction of the track itself. A standard section of the contact rail is 150 lb. per yd. The insulators on which the contact rails rest and the brackets for the contact rail protection boards are supported on the longer ties placed for that purpose. Insulated joints are installed in the track at the time of track laying.
The necessity of providing for positive and negative cables, lighting, escalator and circuit breaker connections requires that a large number of conduits be placed in the trackway. These conduits are all placed in advance of the concrete work. In this manner, delays and interference are reduced to a minimum. Provision is also made for the installation of signal and interlocking apparatus to be installed under other contract.
Rails bolted together and laid loosely in the trackway.
Jacking up the rails for placing ties and tie plates.
Driving screw spikes in prebored holes. Short ties are 2 ft. 6 in., and long ties for third rail support are 3 ft. 1O-1/2 in.
Leveling and spacing ties.
Concrete mixer obtains batches of sand and coarse aggregate properly proportioned from truck with compartments for each batch.
Galvanized iron chute conveys concrete to side-dump cars driven by a gasoline locomotive.
Concreting begins at both ends of section and progresses toward the center.
Spading and troweling concrete in the trackways.
Checking line, grade and gage of track before the first set in the concrete.
Checking for car clearances with full-sized template.