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.