Trains have been blaring their horns 144 times a day for a year as the A Line and freight trains pass through Stapleton area crossings and other residential areas. A year ago, neighbors thought the A Line opening would bring quiet zones and an end to 18 freight train horns a day. Instead the horns have increased eight fold.

What is the problem, exactly, and why is it taking so long to fix? The explanation is complicated, so we offer a simplified version first. For readers living with the noise and hungry for a better understanding, we researched the technical details.

The short version:
RTD says the problem is not train safety. The crossings are operating safely (and are backed up by crossing guards and horns) and the trains are operating safely. Positive train control (PTC) is in control of the trains 95 percent of the time and there are two back-up systems that make it “fail safe.”

The continuation of horns is occurring due to a federal requirement that the wait times at all the traffic crossings is constant so that motorists don’t get frustrated or confused by erratic crossing closures. Due to train speed variations or wait times at stations, the arrival times at gates can vary, and the prediction software has not been accurate enough to ensure constant warning times. Data is being collected from train operations thus far to refine the system so the timing is consistent at all crossings.

RTD is encouraged enough by recent performance improvements to indicate, off the record, that compliance with FRA requirements can be achieved by early spring. The agency says the line has experienced only four major disruptions since opening April last year and has achieved a “daily average on-time performance of 87.3 per cent.” RTD continues to impose a monthly penalty on DTP of $250,000 for non-performance under its concessionaire agreement and says the operator is motivated to solve the warning time problem as soon as possible.

To understand why it’s taking so long, read on.

The technical explanation:
“Iterative refinement of software” is beginning to reduce the “extended gate down times” at the A Line’s 12 crossings with local streets. Between December and the end of January, RTD reports a 15 percent increase in the number of gate closures that remained within 10 percent of the target goal, from roughly 50 percent of the time to 65 percent.

For example, a street crossing with a design warning time of 30 seconds would satisfy regulators’ requirements if the warning time fell within 27 to 33 seconds 90 percent of the time. “Warning time” is defined as the elapsed time between when crossing warnings begin (bells clang, lights flash and the crossing gates descend and close, securing a crossing) and when a train arrives at the intersection itself.

Despite this recent improvement, RTD does not anticipate getting out from under waivers enforced by the Federal Railroad Administration (FRA) for another 60 to 90 days. Those waivers allow RTD to operate the A Line without certification of its positive train control system (PTC) by the use of horn soundings and “crossing attendants” (flaggers). The current waiver expires April 30.

Once the A Line PTC is certified, applications by Denver and Aurora for a quiet zone along the commuter rail line can be accepted by the federal government. RTD general manager Dave Genova has said previously that he expects the FRA to approve the quiet zone applications within 60 to 90 days of submittal. Once the quiet zone is approved, all trains in the A Line corridor, including freight trains, can pass through the street crossings without sounding their horns (except in case of emergency).

RTD is attempting to create the first PTC designed from the ground up for a new rail service. Congress mandated PTC in 2008 and railroads have until 2018 to comply. PTC is designed to eliminate the possibility of train-to-train collisions, excessive speeds, or dangers for railroad work crews. Through GPS and wireless communication, the system continually tracks a train’s location and speed.

This contrasts with the basic design of automatic train control (ATC), the traditional train signaling system. ATC uses fixed “blocks” of track circuitry for detecting a train’s location and establishing maximum train speeds by block. RTD uses ATC as the PTC back up.

RTD Systems Lead Jeff Whiteman describes ATC as “good and reliable but not very smart.” At a media briefing last month, Whiteman provided this example of an ATC block: it might be as long as two miles, the distance required to stop a train traveling at 79 miles per hour. Once the circuitry in that block is triggered by the train’s arrival, the warning time for the upcoming train crossing is set and cannot be altered.

The system works well for technology such as freight trains with their relatively constant speed, infrequent stops and huge mass. The system is less efficient for commuter rail transit characterized by frequent stops and starts, variable speeds and variables introduced by the behavior of train operators, motorists at crossing and patrons at platforms (e.g., pedestrians crossing in front of a train attempting to leave a station).

ATC and PTC both are grounded in the premise of prediction – attempting to predict when a trail will arrive at a crossing. Prediction is necessary because of the “braking curve”, the extended period of time required to stop a train of a certain mass traveling at a given speed. Simply stated, it can take a long time and distance to stop a train.

PTC, with its foot-by-foot knowledge of where a train is and how fast it is traveling, aims at reducing the gate down time that would otherwise be established by an ATC. If the A Line ran with ATC as the default signaling system, it estimates the 37-minute trip from downtown to the airport would run seven to eight minutes longer. But it has to do so while also maintaining a “constant warning time”. That constant warning time is deemed essential by RTD and the FRA so that motorists don’t get frustrated or confused by erratic crossing closures and attempt to “beat the system” by racing trains to the crossing or slaloming around gates in the absence of a train.

The basic problem is that the prediction software has not been accurate enough to ensure constant warning times. RTD and its operator, Denver Transit Partners, have been going through a process of “iterative refinement of software upgrades based on recent data.” The data pertain to the three behaviors identified above as well as two additional complications: “train meets” (where two A Line trains travel through a crossing relatively soon after one another) and when a freight train crossing is added into the mix. Additional complications arise when street crossings are in proximity (e.g., Dahlia St.-Holly St.-Monaco Pkwy.) or when stations are on the “near side” of a crossing (e.g., the Ulster St./Central Park station for eastbound trains). Without a “constant warning time”, crossing gates would fly up and down in rapid succession confusing and frustrating motorists.

At its media briefing, RTD emphasized that PTC is the default control system with ATC as a back up. Both systems allow for “enforced braking” that override an operator when design parameters are exceeded. RTD says both systems will “fail safe.” There is even a third method of operation – “stop and proceed” whereby an operator would have to stop at each crossing and await a notice to proceed delivered remotely from the agency’s central Operation Control Center.