Go No Go: Cab Signaled Transit Wayside Dialects

In the late 1960's a new crop of "Space Aged" rapid transit systems began to take shape in North America making use of the new materials and electronics developed since the end of World War 2. In particular was a push to replace wayside signals and trip-stops with cab signals which would enable both automated operation and a reduction in wayside hardware. Of course this forced the question about contingency operations in case of cab signal or related failure. Railroad explorations of wayside elimination in the 1930's and 40's had retained wayside signals at interlockings and other controlled points and rapid transit followed suit with the further innovation of reduced aspect signals displaying Stop and Proceed aspects with the occasional Diverge and Absolute Block. Also like railroads, the rapid transit systems adopted a mix of "dialects" for their reduced aspect signals that I hope to categorize below.

Lunar White - The Granddaddy of them all, the Lunar White proceed indication can be considered the "default" rapid transit proceed signal aspect. First appearing on the PATCO Speedline in South Jersey (at least as far as I can tell), lunar proceed was later adopted by systems including DC Metro, SEPTA MFL and Route 100, HBLR, Cleveland Rapid Transit and Baltimore Metro (as built). The rationale was to be distinct from the existing ABS signal aspects using Green and Yellow and present as a railroad Restricting style aspect for non-cab signal equipped movements.

PATCO Lunar White Cab Speed

Steady Green - Considered the "obvious" solution since Green means Go, use of Green in place of Lunar White has been gaining in popularity with newer systems as any perceived need to be distinct from older ABS systems has faded. Notably appearing on DC Metro peer system BART in 1972 it was also adopted by Maimi MetroRail, St Louis Metrolink, Baltimore Light Rail and the Baltimore Subway as modified. Some systems will use flashing green to indicate a diverging route, others yellow. 

Baltimore Metro Steady Green Cab Speed

Flashing Green - Taking another page from the railroad playbook, flashing has made a few appearances to indicate a proceed indication on rapid transit systems. On the New York city subway flashing green straight up means Cab Speed for both straight and diverging movements under the control of the CBTC system. On Atlanta's MARTA, flashing green is the default proceed signal with steady green indicating a diverging route. 

Yellow - Similar to lunar white, this substitutes lunar for yellow similar to that dialect of railroad Restricting indications. This is most prominently used in Boston with Y/R for straight routes and R/Y for diverging.  

MBTA Y/R Cab Speed

Green Arrows - This most prominently appears on the cab signaled  portions of the Dallas DART system to differentiate from the ABS signals and avoid the use of flashing. 

DART arrow signals, not illuminated.

 

DART arrow signals, illuminated.

White Arrows - Like the green arrows above, but using either lunar or plain white. This is popular with airport people movers including the JFK AirTrain but also in use on the Sound Transit light rail with stylized direction indicators.

Sound Transit stylized arrow.

ABS - Currently used on Chicago, this method of go-no-go signaling takes a cue from the cab signal state to display a Green if the cab speed is "clear" (55/70mph) and a yellow if it is "restricted" (35/25/15mph). The cab signals can be from either block state or due to civil speed restrictions.

CTA Proceed Clear

 

CTA Proceed Restricted

This is my best shot at a taxonomy. I'm sure I've forgotten about a few systems or corner cases so if you, the reader, can think of any, please let me know in the comments. Please make sure that the line is actually cab signaled as there are quite a few ABS signaled light and heavy rail transit systems in North America.