This is a summary of dates I have come across while trolling the internet.  Some of them may be less reliable than others, and I have seen discrepancies of a year or two in some statistics… But for the most part, the dates seem to be accurate.  A huge amount of research has been done by people such as J B Calvert, and the web-pages they have (listed below the dates), should be read by anyone wanting an in depth discussion of the signals referred to in this list.  Obviously, one can put milestones on the list to cover just about every railroad, so if you know of any significant milestone that should be included, please send it to me!.....

1828? - The Liverpool and Manchester RR in England is given credit to have used the first form of "signaling", having used policeman to stand at intervals along the right-of-way to signal passing trains as to the occupancy of the track ahead, in addition to keeping people off the track (4)(P.17)

1834 - Again, it is generally believed that the Liverpool and Manchester was the first to use fixed wayside railroad signals, consisting of a post with an iron bar holding a flag, which could be rotated to show if the track ahead was occupied or not - if the flag was parallel to the track, it meant the track was clear… perpendicular, it was not. (4) (P.17)

Mid 1830's - The first ball signals were used on the New Castle and Frenchtown RR (in Delaware)… they actually started off using flags, but they were replaced when the railroad figured out the flags weren't always going to fly properly when there was no wind.  They were replaced by cloth covered peach baskets, inverted, and were used to monitor the progress of a train. (1)(P.134) also (4)(P.17) (neither reference comes up with a more exact date)

1837 - Ball signals introduced to England on the Great Western Railway.

1840 - A "disc" signal was installed on the New Castle and Frenchtown RR. This signal had a four foot diameter red disc with the word "danger" painted on it, and rotated 90 degrees so the sign could be seen or not seen. (1)(P.136)

1841 - First semaphore used on the London and Croydon Railway in England (1)(P. 169)

Late 1850's - England started interlocking mechanical and signal controls so their movements would succeed each other in a predetermined order….. Imported to the U.S. in the mid 1870's

1860 - The "gate signal" was developed as a crossing signal to protect the intersection of two railroads. The gate could swing to block either of the tracks, and could include a lantern for night time operation. (1)(P.136)

1860's - the Smashboard signal was developed expressly to prevent entry to an open drawbridge (1)(P.136)

1863 - First block system came into use on the United New Jersey Canal and Railroad Companies between New Brunswick and Philadelphia PA

1865 - In March, the Franklin Institute appointed a committee to investigate sound signals used on the railroads, and then in September a survey was taken to include "color signals" (1)(P.130)

1865 - The Reading and Philadelphia RR used white/blue/red to signify proceed/caution/danger

1867 - Thomas Hall invented and patented a "switch signal and alarm" (1)(P.130)

1869 - Automatic electric block system invented by Thomas Hall (1)(P.128), and the "Banjo" type signal was invented by him (4)(P.36)

1870 - First interlocking plant - Trenton NJ on the United New Jersey Canal and Railroad Companies, built by Saxby and Famer of London….
American rights for Saxby & Farmer was secured by Toucey & Buchana Switch Co of Harrisburg PA (1)(P.169)

1870's - The Pennsylvania RR, well known for a long list of "firsts", was among the first to use semaphore signals for interlocking. (4)(P.39)

1871 - The Hall Signal Co was founded by Thomas Seavey Hall (this is according to a reference to Brignano and McCullough in "The Search for Safety", a publication put out by US&S). (4)(P.36)

1871 - Pennsylvania RR absorbed the Camden & Amboy, and block signal installation was completed between Jersey City and Philadelphia

1872 - The basic track circuit was invented by Dr. William Robinson, but not used in any extent till after 1880

1873 - Franklin Pope formed the Electric Railroad Signal Company to market his "repeating electric semaphore", which was really a disc type signal. (1)(P.130)

1873 - Daniels Rousseau's automatic signals installed on 3 miles of the NYNH&H, extended to 16 miles by 1876, but soon abandoned due to fatal flaws (2)

~1874 - The "crossing target" or "tilting target " signal was invented on the Pittsburgh, Fort Wayne and Chicago RR. (2)

1875 - The Boston & Albany RR had equipped 44 miles of track between Boston and Worcester with Hall disc signals. (4)(P.39)

1875 - An organization met in Chicago, thereafter called the General Time Convention of Railway Managers, and recommended adopting a standard time for the railroads (1)(P.129)

1878 - Dr. Robinson founded the Union Electric Signal Co

1881 - George Westinghouse founded US&S by consolidating the Union Electric Co (holder of closed track circuit patents) and the Interlocking Switch & Signal Co (holder of interlocking patent rights)

1882 - The Pennsylvania RR installs its first electro-pneumatic lower quadrant semaphores. (4)(P.39)

1883 - Standard Time was adopted, but not universally accepted right away (1)(P.129)

1894 - Delaware, Lackawanna, & Western installs fixed signals (1)(P.128)

1884 - The Pennsylvania RR is the first to install an automatic block network network utilizing electro-pneumatic lower quadrant semaphores between East Liberty and Wilkensburg PA - by the end of 1884, 65 were in service. (4)(P.39)

1891 - Frank Patenall goes to work for the B&O RR after working for US&S, he came to America in 1885 from England, where he worked on signal systems there. (4)(P.60)

1890's - Hall came out with a modified Banjo signal which had the cloth painted with a semaphore like looking blade, to combat the steady increase in the usage of semaphore signals (1)(P.149)

Late 1890's - Hall developed a "rotating semaphore" signal, where the blade was attached to a motor, and as long as the blade was moving, the engineer could proceed, if it was not, the engineer had to stop the train. At night, the signal was lit by a red light light, and if it was steady, the train had to stop, if it was flashing, the engineer could proceed. Tested successfully on the Delaware, Lackawanna, and Western RR, but never used much. (1)(P.149)

1892 - The Board of Trade in England required the use of green instead of white for "proceed". (3)

1892 - Controlled manual block system installed in the Park Avenue tunnel (NYC&HR - New York Central and Hudson River RR) - imported from England - called the "lock and block" or Sykes" system after its inventor - installed by US&S. (2)

1893 - The CNW RR replaces white with green for proceed in their 1893 rulebook, pre-dating the NYNH&H change in 1895, to whom credit is usually given to as being the first. (3)

1895 - A wig-wag style signal was invented by D. H. Wilson in February. (8)

1898 - The Bezer Rotating Signal was invented, and was tested on the Delaware, Lackawanna and Western at Kingsland NJ in 1896, but was never seen or used anywhere else. (8)

1901 - From a questionnaire sent out by the Railway Signal Association, it was found that the average distance from the distant signal to the home signal was 1444 feet, and to the interlocking tower was 1750 feet

1903 - The Patenall three aspect upper quadrant semaphore signal (invented by L. F. Loree and Patenall this year) was adopted as standard by the ARA, and was the majority of new installations till 1940. (2)(3)

1903 - First AC track circuit and vane relay - US&S

1904 - June 13th, GRS was formed from the merger of Pneumatic Signal Co of Rochester NY, Taylor Signal Co of Buffalo NY, and Standard RR Signal Co of Arlington NJ. (GRS/Althom website)

1904 - It was reported in a 1908 article in Railway Gazette magazine, that there were 4,697 "enclosed disc" type signals employed in automatic block signaling service, the majority of which were manufactured by Hall. (4)(P.39)

1906 - From a questionnaire sent out by the Railway Signal Association, it was found that for 15 roads queried, approx one-third of the mileage in the U.S., the average distance from the distant signal to the home signal was 3745 feet, and to the interlocking tower was 4025 feet.

1906 - The first of the three aspect upper quadrant semaphores was installed on the Pennsylvania RR between W Philadelphia and Elwyn PA. (3)

1906-1908 - Research by Corning into lenses and colors, and they came up with the current green-yellow-red, replacing white-green-red

1907 - The Interstate Commerce Commission established a Block Signal and Train Control Board, it went out of existence in 1912 and it's function handled by the Safety Division.
In 1917 the name of the organization was changed to the Bureau of Safety of the ICC

1908 - Upper quadrant signals became the recommended standard.

1911 - According to his book, the author notes that GRS literature attributes the development of an Absolute Permissive Block system to GRS signal engineer Sedgwick N. Wright.

1913 - The Pennsylvania RR starts electrification. (9)

1914 - Perfection of a concentrated-filament lamp to provide color light signals a satisfactory sighting distance.

1915 - Dr. William Churchill of Corning Glass developed a pale yellow "fog-penetrating" glass, which the PRR then used in their PL signals. (3)

1915 - The Pennsylvania RR introduces (experiments) with the first version of the PL (position light) signal and it is nick-named a "tombstone" signal because the top of the signal was round and the signal resembled an old tombstone…… also, later adopted on it's affiliates the N&W and LV  (9)

1915 - The ARA adopted the idea of classifying routes by allowable speed, hence, speed signaling was developed. (2)

1916 - The PRR and NYC finally change their rulebooks to reflect green being used for proceed instead of white. (3)

1917 - US&S becomes a subsidiary of Westinghouse Air Brake Co (7)

1917 - The Pennsylvania RR changes the shape of the "tombstone" signals slightly so they do not present as much wind loading as the older style… they also used "stepped" voltages of 11, 6, and 4 volts to adjust for day, twilight, and night (and these were manual adjustments made by the signal guys) (9)

1918 - By the end of WWI, white for proceed was out and prohibited by the I.C.C.

1918 - The Pennsylvania simplifies the design of their PL signals, eliminating one of the four "heads" for each aspect (9)

1918 - Hall enclosed disc (banjo) signals were no longer shown in the New York Central RR rulebooks, as well as three arm lower quadrant semaphores (being replaced by two arm upper quadrant semaphores). (2)

1920 - Searchlight signals appeared with a single lens.

1921 - CPL's first used in their modern form on the B&O and affiliates. The first B&O section to get them was between Deshler and Hamlet OH. (4)(P.62)

1921 - The Pennsylvania RR's PL's started taking the shape and configuration were are accustomed to today. (9)

1923 - First inductive train control (continuous cab signaling system) - US&S.

1923 - First industrial use of vacuum tubes - US&S.

1924 - B&O announces in the December issue of Railway Signaling magazine that it had ordered 77 CPL signals for the Staten Island Rapid Transit. (4)(P.62)

1924 - First remote controlled gravity hump yard - US&S

1924 - The CNW installed its first GRS type E semaphore signals. (3)

1926-1927 - The CNW installed two indication continuous cab signals between Chicago and Council Bluffs IA (511 miles), and removed the lineside signals. (3)

1927 - GRS came out with the first Centralized Traffic Control machine, again, invented by Sedgwick Wright.  CTC combines interlocking with automatic block signaling. (4)(P.63)

1927 - The first CTC installation was on the New York Central Ohio Division between Stanley and Berwick OH. (4)(P.63)

1929 - Color light signals made their first appearance in the CNW rulebook, but were arranged horizontally instead of in the more familiar vertical fashion we are accustomed to today (the CNW was the only railroad to use the horizontal arrangement). (3)

1930 - The Pennsylvania RR introduced pedestal signals (9)

1930 - Searchlight signals with compound lenses appeared.

1934 - First coded track circuit - US&S.

1936 - GRS introduces NX Interlocking in March, full description here.  The basic principle of the system hinges on the fact that trains through an interlocking plant travel by routes which have an entrance and an exit.

1940 - Color light signals became the dominant form of signals for new installations. (2)

1942 - First coded carrier current CTC system - US&S.

Late 1940's - The Pennsylvania RR has converted 97% of their signals to position light types (9)

Mid 1950's - Some bright management type decided that the Pennsy PL lights should have two horizontal red lamps for the stop aspect instead of three horizontal yellow, so, now the red indication seemed to "disappear" when they had fog (8)

1953 - The CNW replaces a white marker with a red marker on distant signals. (3)

1964 - The N&W starts colorizing their Position Light's. (11)

1965 - GSX, General Signal Corp was formed to expand and diversify the GRS business, GRS a wholly owned subsidiary of GSX

1967 - The Pennsylvania RR places their first signal to the left of the track it governs, on the Williamsport-Buffalo line, also placed 2 PL signals on a bracket post at CP-Lake IN (10)

1968 - American Standard purchases US&S and makes it a separate division (7)

1981 - First computer planned and controlled train meet, courtesy GRS

1980's - The semaphores on the New Haven come down.

1988- Amtrak starts the colorization of the Pennsy PL's (10)

1988 - US&S acquired by Ansaldo (Italy) (7)

1989 - GRS acquired by Italian Co Sasib and joined the Sasib Railways group, the GRS name was still used

1993 - Rochester no longer the HQ for GRS, admin and engineering moved to Brighton, and manufacturing moved to Sawgrass…. Sawgrass closed in 2003

1998 - "GRS" becomes part of Alstom when it acquired Sasib, and the GRS name went away

Jan 1, 2009 - US&S becomes a wholly owned subsidiary of Ansaldo Signal N.V., a Finmeccanica Co

1998 - CSX replaces B&O CPL's between Cumberland MD and Pittsburgh PA on the famous Sandpatch grade… one of the first such replacement efforts noted. (6)

2000 - The CSX takes down the semaphores along the Monon route in Indiana and auctions them off at a closed and secure location in Rushville IN on July 8th.

2002? - The last operating ball signal in New Hampshire is taken down.

2005 - The last semaphore on the Southern Tier in New York comes down.

2006 - Maryland's last semaphore in Dundalk MD, a fixed signal, is replaced by a color light signal.

2007 - The mechanical semaphore interlocking at Brighton Park Junction was taken out of service and replaced with modern color light signals. Flagmen were also required along with the semaphores.

2012 - The last known "full" CPL comes down in Baltimore at Carroll Interlocking south of Bayard Street and Bailey's Wye.

Friday, June 15th, 2012 - ALTO Tower in Altoona PA closes.

2016/2017 - Pennsy PL signals fall along Norfolk Southern's Harrisburg Line between Duncannon and Altoona.
I know schools don't like to have their students use Wikipedia as a reference, but it's so convenient… with that said, here's where I found my info:

(1) Trains and Technology: The American Railroad in the Nineteenth Century by Anthony J Bianculli
(2) New York Central RR Signals 1912-1918 by J B Calvert: http://mysite.du.edu/~etuttle/rail/nychr.htm
(3) The Chicago and North Western Distant Lamp by J B Calvert: http://mysite.du.edu/~jcalvert/railway/cnwdist.htm
(4) Railway Signaling, a book by Brian Solomon
(5) The Elements of Railroad Engineering by William Galt Raymond… seems to cover just about everything, not just signals, as it lacks the details other books contain
(6) Semaphores dot com website: http://www.semaphores.com/semaphores/resources/lists/BNOCPLs.html
(7) Wikipedia on US&S: http://en.wikipedia.org/wiki/US%26S
(8) Great discussion about early signals: http://mysite.du.edu/~etuttle/rail/sigs.htm
(9) More PRR T&HS stuff: http://broadway.pennsyrr.com/Rail/Signal/aspects_us_pl.html
(10) Good historical reference site : http://mysite.du.edu/~jcalvert/railway/railhom.htm

Another good "old" book: Train Operation by William Nichols: http://books.google.com/books?id=44M5AAAAMAAJ&pg=RA1-PA157&lpg=RA1-PA157&dq=position+light+signal&source=web&ots=4iPgWV4mo8&sig=gIENIp_ESZeGTxMU7HpKkYwrgHU#PRA1-PA156,M1

Another good signal reference book: Railway Signaling by Everett Edgar King (1921): http://books.google.com/books?id=I4Q5AAAAMAAJ&pg=PA14&lpg=PA14&dq=position+light+signal&source=web&ots=4OYZr-Kisf&sig=TblDwCGux5zMvNbHi-3LVkiq9Hw#PPA11,M1

Wikipedia article on railway signaling: http://en.wikipedia.org/wiki/Railway_signalling

Reference: http://books.google.com/books?id=I4Q5AAAAMAAJ&pg=PA1&lpg=PA1&dq=railroad+signal+history&source=web&ots=4OYZoVGjtc&sig=8C3OiL_wWH8x1WEQXd2CsQToEdk#PPA7,M1

Interesting site on signals at Worcester Shrub Hill (U.K.): http://www.roscalen.com/signals/Worcester/ShrubHill.htm

Like traffic lights?, check out: http://signalfan.freeservers.com/history.html

Unfortunately for us, everything by CG Tower has seemingly disappeared from the internet, a sad loss for us all......  Because of that, I have started to archive pages I think are relevant and important.

Code cleaned up and Links last checked Nov 20, 2016.

NEW 12/15/2007
Last Modified: 10-Feb-2017