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.

MUNI Metro Subway - Unrealized Capacity

You may have read about MUNI's radical attempts to deal with congestion issues in its Metro Subway that runs under Market Street and also included the Twin Peaks Tunnel.  Long story short, MUNI is eliminating one seat rides downtown for riders on the J, K and L streetcar lines.  The given reason is since the J and K lines are limited to single unit LRV operation, those "slots" in the Metro Subway are being underutilized and the new operating plan will replace the one LRV trains with two LRV trains. 

The Metro Subway is signaled by a loop antenna based CBTC system in the style of LZB and if you are noticing a pattern between articles addressing CBTC and capacity problems then I thank you for being a long time reader.  Basically MUNI is noticing the capacity problems that stopped both SEPTA and MBTA from realizing a full CBTC fantasy in their respective trolley subways and MUNI's response is to make many commutes much worse.  To be honest this isn't just a CBTC problem as coded track circuits would have been no better and possibly worse.  The issue is a fear of less automated operation.

Here is an LRV on the eastbound track at the Embarcadaro terminal station, which seems to be the major capacity constraint as M, L, K and J line trains all turn back here.  You might notice a line of cones and a lot of unused platform space.  That is because at every Metro Subway station, only one train can platform at a time, even though the platforms are long enough to support two trains.

Here is the westbound track with a fresh train sitting behind the cones just hanging out with a second train close behind while they wait for the single loading/unloading berth to become available. On all of the Metro subway stations it is common for following trains to stop short on the platform and wait for the single loading zone to become available.  It is also common for passengers to run their buts off along the platforms to reach said single loading zone from the far end.

Both SEPTA and the MBTA use multiple berths at underground trolley stops to varying degrees.  For example at Juniper St there is an unloading spot and a loading spot.  At other stations different routes can stop at different points along the platform.  On both systems the signaling system is equipped with R/Y station signals that allow operators to creep forward and occupy the station behind another LRV.  It's not a cure all, but it helps. 

MUNI plans to update its CBTC system to one that uses wireless instead of loop antennas.  It might work better, it might not, but with new LRV's already arriving, maybe someone should have thought outside the box and ordered a radar based collision avoidance system to allow closer spacing in stations and thus pipeline the passenger boarding operation.  Once headways drop below two minutes, dwell time and terminal capacity dominate block separation.  It's why expensive CBTC systems don't move the capacity needle much and often do worse than traditional systems with on-sight operation, spring switches and loops.

CTA Mini-Towers

The Chicago Transit Authority has remained a strong supporter of the "tower" system of rail transportation control.  Just like the NYC Subway and the Long Island Rail Road, the CTA's traffic density still gives an edge to human eyes and judgement in place of automated routing and remote control.  Those who ride the CTA are probably aware of the currently manned towers placed at the yards and major junctions.  It is pretty hard to miss them the way the operator's cabs typically cantilever out over the tracks.

The CTA even produced this video showing its recently rehabilitated towers at TOWER 18 and CLARK JCT, which received brand new NX Panels.



However the CTA has another type of tower, one  that is almost ubiquitous, but also hard to spot.  While local control panels are a common feature in most relay rooms, the CTA steps things up one notch by placing many of these, from junctions to simple crossovers, in their own little booths.




 The most "tower" like of these mini-towers is TOWER 12 at the southeast corner of the loop where Orange and Green line trains split off to the south.  While this junction is normally remote to Tower 18, there is still a small porta-potty sized booth where a human operator takes over at peak periods.

Another form of the CTA "mini-tower" is a space built right into the trackside relay huts.  Again the giveaway are the windows placed in a manner that would allow an operator full view of arriving trains.

Here is a portable interlocking cabin set up at the Harlem and Lake Green Line terminal and again the small window for manned operation is present (also note the interlocking horn).



Finally, sometimes the operator can get his own stand-alone booth on the station platform.  One such mini-tower is located at the south end of the Damien Blue Line platform.

This mini-tower controls a small scissors crossover immediately behind it.

The interlocking is clearly remote from somewhere else, but as you can see the Damien local tower has a fully functional panel.

 It has NX buttons for the signals as well as overrides to throw the switches without a signaled route.  It also has buttons for the direction of traffic on all four exits, call-on buttons and even a button to sound the interlocking horn.  Of course there are switches to set control local, remote or automatic.

What's interesting and a touch ironic is that at the opposite end of the same platform is a classic style tower that was built, but never placed in service as the junction it was to control was eliminated.  It looks like the CTA took a second bite at the apple. :-)