Line of Sight - DART's Third Method

Last year ago I wrote an article on how the DART light rail system in Dallas used two distinct signaling methods.  Automatic Block Signals with Automatic Train Stop  were used on the original Red and Blue lines built in the 1990's and audio frequency Cab Signals without intermediate wayside signals on the Green and Orange lines built since 2000.  However there is a third method first used on the initial segments and most recently employed on the Blue Line extension to Rowlett. This method is Line of Sight operation and while I had seen the signs, I did not fully learn what it signified until I recently had the opportunity to ride the southern portions of the Red and Blue lines that make the most use of Line of Sight.

Line of Sight territory is exactly what it says on the tin.  Trains must proceed able to stop short of any obstruction including other LRV's, pedestrians, motor vehicles, traffic lights, etc.  It is governed by the following section in the DART rulebook.

Line-of-Sight Operation: 

4120 Line - of -Sight Territory Incorporates mainline tracks not protected by ABS/CST. Line-of-sight territory incorporates the following:

• West Oak Cliff Branch - Zoo Station to end of line at Westmoreland Station.
  
  
• South Oak Cliff Branch - Morrell Station to end of line at Ledbetter Station. - Central Business District - Houston Street to Pearl Station.
  
  
• Rowlett - Signal G1813 to Rowlett Station 

4121 Line of Sight Speed Trains must operate at a speed which will enable the train operator to stop short of any obstruction. All special instructions or posted speed zones must be complied with. Speed shall be reduced when adverse operating conditions exist, (i.e.) poor weather. 

4122 Minimum Separation Trains must maintain a minimum separation of at least two LRV lengths unless authorized by the controller or when a train is stored in the tail track. a. No more than one train at a time in the same direction may occupy a city block in the CBD unless authorized by the controller.

When I first encountered Line of Sight Operation at the Garland station on the Blue Line it was accompanied by a two aspect signal that I surmised was part of a low cost go/no-go ABS signaling system.  From the overhead and street view I could identify other signal signals along the line and it seemed that LRV's would approach each prepared to stop within Line of Sight.

It turns out I was only partly correct as these Block Indication signals have their own alternate method of operation. On the two southern routes, Block Indicator signals can work in conjunction with Start Block (SB) and End Block (EB) signs.  Passing Block Indicator with an SB triangle the train is protected by track circuit block until the following EB triangle. I am not entirely sure if the SB is needed before any EB, but that seems to be how it was set up.

The Block Indicators are used where there are line of sight issues like curves and steep overpasses so that LRV's don't have to crawl on these stretches of track.  On the straight and level trains are able to travel on line of sight up to a speed of 45mph. This is of course quite safe as the LRV's are designed to run in traffic and have magnetic track brakes for bus like stopping. When exiting the ABS territory that proceed the Line of Sight territory LRV's receive an "Approach" class indication before the LoS territory sign.

SEPTA two aspect block signals on the Route 102

The DART Line of Sight system is similar to the two aspect signals used on SEPTA's Routes 101 and 102 suburban trolleys, although in that case the signals partly work as a more formal two aspect ABS system. DART Line of Sight allows higher speeds on stretches of track located on median rights of way or city streets governed by traffic signals.  It is worth noting that at least at the time of writing DART trains are scheduled such that only one LRV is expected to be in any LoS section at a time. For example a Red Line train departing Westmoreland is scheduled to enter ABS territory in 11 minutes and the maximum headway is 15 minutes. 

DART's Line of Sight system is a safe and innovative way to achieve cost reductions and enable Light Rail style in street or street adjacent operation with traffic and other non-interlocked signaling. Thanks to the straight jacket of PTC, non-FRA regulated light rail systems are the only place that are able to employ outside the box signaling and safety solutions.

DART - One System, Two Methods

Dallas Area Rapid Transit is one of the new generation of North American light rail systems, as large in scope as many heavy or commuter rail networks, but built to a much lower price point.  DART was built in two waves, the initial Blue and Red lines built between 1996 and 2002 and then the Orange and Green lines built roughly between 2010 and 2012.  Despite being part of a unified transportation system, DART uses two completely different signaling methods.

 Signaling on the Red and Blue lines consists of pretty standard ABS CTC with wayside signaling.  Signals consists of single direction stub masts placed back to back.  An ATS capability is provided via some sort of IIATS or Westcab transponder to enforce signals displaying Stop, however track speeds are not enforced.

Interlocking signals use a NYC Subway style system with the upper head indicating block state and the lower head route.  Thus G/Y is diverging clear and Y/G is approach straight.  Also note the use of traditional track circuits with impedance bonds.

Of course for the second phase Orange and Green lines, DART decided to switch gears and adopt a cab signal system without fixed wayside signals.  Cab signals are of the jointless audio frequency type and signals are only provided at interlockings with the ability to display Stop (R), Restricting (R/Y) and lighted directional arrows for everything else.

These directional signals were also used in the Phase 1 street running portion downtown, so it was natural to reuse the system in cab signal territory to reduce the need for re-training.

Unfortunately my photo of the cab signal display did not turn out, but there is not a lot of complexity.  The operator sees the vehicle speed in red LED and a target speed in orange LED.  The speed changes as soon as a new cab signal code is received with an ATO style brake application being instantly applied, as opposed to a penalty application after a grace period. This system enforced both track and block speeds with speed control in 5 or 10 mph increments between 10 and 60mph.

It was interesting to see these two generations of systems running side by side with Orange line trains in particular starting under one system and finishing under the other.  It was also interesting to see how much train operation was being impacted by the speed enforcement with operators having to brake about 5mph past the speed target and then re-accelerate.  This created a very jerky form of operating where as the lines without speed enforcement saw much smoother operation.

Although many light rail systems are adopting or converting to heavy handed forms of train control, there are still many systems built since 1990 that have retained lower cost, human-centric signaling systems that will likely be the last such examples as heavy and commuter rail lines become increasingly "supervised".

EDIT: DART actually has a THIRD method of operation that I discuss in a followup post.