Hello West Side Joe
Thank you much for your nice and supportive comments. Semaphore signals were used initially on the Manhattan (Manhattan Railway Co.) and Brooklyn (Brooklyn Rapid Transit)_ Victorian Era EL's under steam engine operation and later electrified operation. When the first subways opened (IRT) the dark tunnels called for a colored illuminated lens signal lamp style. That is how the beginning of the following various types and styles of modern era "illuminated" signals became the standards, Many El lines retained semaphore signals -- the four Manhattan EL's had NO signals except ONLY at sharp (and blind) curves and at all interlocking - and on later added two-way operation center single express tracks (block signals). The subway line "EL" extensions started with semaphore signals in 1904-8 era but over the decades these were in many cases replaced by modern lighted color aspect signals.
Semaphore signals BLADES gave excellent visibility in daytimes - and the signals also had lighted aspects for evening visibility -- one bulb that illuminated one of 3 colored discs on a fixed plate that rotated with and matched the semaphore blade position of either go, slow, or stop positions. I provided BELOW some (of quite many) photos I have of most EL lines with semaphore signals in use. The Chicago and Philadelphia EL's used semaphores also.
When I started my O-Scale NYC EL layout back in 1985, there were no O SCALE "post war" modern subway cars models produced --- so I had all pre war IRT and BMT EL Gate and MUDC cars, and a number of pre war IRT Subway cars. So I modeled the 1930's thru 1960 era -- with Manhattan & Brooklyn trolleys operating under my EL. And as for signals -- I provided both Semaphore Signals and standard lighted aspect (modern) signals -- which I grew up with from mid 1940's thru 1960's. Most semaphore signals were gone from surviving BMT and IRT EL lines by late late 1970's --- tho a few (like at New Lots) here and there survived into the recent years until removed.
Below are some prototype examples -- a few of many hundreds on images of them on the EL's; The very high-post semaphores (blades) were used on (and governing) express tracks to clear the roofs of local trains that passed by ---------------
The 1st photo is a recent era image of a still working semaphore is at the yard approach lead track from the IRT New Lots Ave. Terminal Station (over Livonia Ave) -- this signal no longer exists -- a signals relay control room was built on the structure where it was located --
The 2nd (left) photo is on the IRT 9th Avenue EL at the Rector St Station - one of the rare signals on a station platform
The 3rd (right) photo is one I took from the rear of an uptown 3rd Ave EL MUDC-class Local back in late 1954 of semapore signals at interlocking on the (Bronx) 3rd Avenue EL just north of the E. 161st Street local station. Boston Post Road enters at an angle at left. These signals and switches were removed by 1957 when the center track was deactivated after having no more express service starting 8:01 PM AM Thurs. 5-12-1955 due to the then closing of the Manhattan 3rd Ave EL service below E. 149th St Station.
The 4th photo is looking north from rear car of a downtown IRT Woodlawn-Jerome Ave train about to enter the E. 167th Street Station -- with 170th St station in distance.
The 5th Photo is taken from the same location as the 6th photo - south from about E. 164th Street & River Avenue, but showing two IRT 9th Ave El trains of El Gate Cars
The 6th photo is same location as 5th photo -- but with two IRT Subway trains of the Woodlawn-Jerome (today # 4) line
The 7th photo is -- well, you know where it is I am sure .... -- Regards - Joe F
NOTE: After first posting this - I had to re-edit this twice -- as the photo orders were scrambled and some missing ...