A 1912 description of the Dual Contracts expansion published by the Public Service Commission.
A 1913 guide to the Dual Systems period of subway expansion, by the New York City Public Service Commission. Outlines the Dual System plans, routes and stations of the new lines, contract costs, etc. (6 chapters)
A series of articles published by Engineering News in 1914 detailing various aspects of the construction of the Dual Contracts subway lines. (12 chapters)
This 1918 book by James Blaine Walker details the politics behind the development of New York's elevated lines, its first subway, and the Dual Contracts.
In 1924, B.M.T. Chairman Gerhard M. Dahl published this book (an expansion of the above articles "Build More Transit Lines" and "Rush Hour Relief For Passengers") as propaganda in the fight to get Mayor Hylan to expand transit lines in New York City.
This 1960 article from the ERA details the history of the Steinway Tunnel: Ground broken in 1892, and not used for subway service until 1915, this tunnel was an important link in the Queens portion of the Dual Contracts.
Engineering News, March 10, 1910. A 1910 report on the so-called Tri-borough system, which was the genesis of the Dual Contracts. The Tri-borough plan included the Centre Street, Lexington Avenue, and the Fourth Avenue (Brooklyn) subway lines.
William Gibbs McAdoo's 1911 offer to build the Triborough System.
Public Service Record, June, 1915.
Public Service Record, November, 1917.
Public Service Record, October–November, 1918.
Electric Railway Journal editorial criticizing Hylan and the Board of Estimate for problems completing the new subway lines of the Dual Contracts.
Two 1924 articles about the need for completion of the 14th St.-Eastern (Canarsie) and Nassau St. lines, and overcrowding on the Williamsburg Bridge routes.
New York Times, June 19 & June 20, 1915.
Public Service Record, June, 1915.
Public Service Record, July, 1917.
Public Service Record, September, 1917.
New York Times, January 1, 1918.
Electric Railway Journal, January 12, 1918.
New Line Supplements the Existing Extension of the Interborough Subway and Affords a Direct Route Without Transfer Crosstown and Downtown From Queens Borough. (New York Times, March 16, 1919.)
Man Shot Through River Bed From [Montague Street] Tunnel Was Never Squeezed So Tightly Before. Kept His Mouth Tight Shut. Body Of Third Victim Found-- Damage To New Subway More Serious Than Was Thought. (New York Times, February 21, 1916.)
New York Times, March 16, 1919.
A 1915 description of the engineering effort involved in building the IRT West Side Line's Harlem River Tubes, with many illustrations and photographs.
Scientific American, July 1918.
Public Service Record, July, 1918.
New York Times, August 2, 1918.
Public Service Record, June–September, 1918.
Electric Railway Journal, August, 1918.
Public Service Record, December, 1918.
Scientific American, 1919.
New York Times, March 16, 1919. Also from the New York Times: Clark Street Tunnel Ready (April 11, 1919), Clark Street Tunnel Tested (April 12, 1919), Fear B.R.T. Strike- Rush Use of Tunnel (April 13, 1919), New Subway Service Between Brooklyn and Manhattan Boroughs (April 13, 1919), Open Clark Street Line (April 16, 1919), Lessens Subway Crush (April 17, 1919), Dual System is Nearly Completed (May 4, 1919).
Public Service Record, December, 1916.
New York Times, June 20, 1915.
Electric Railway Journal, November, 1916.
Public Service Record, March, 1917.
Public Service Record, June, 1917.