A Red Line train recently came apart. What happened?

Riders faced a roughly five-hour service suspension on Metro’s Red Line between Gallery Place and Rhode Island Avenue Friday after two cars of a train pulled apart unexpectedly. One hundred and eight passengers were aboard the 8-car 6000-series train during the ordeal, and DC firefighters evaluated two people for possible injuries.

The Red Line train which came apart had left Union Station and was heading outbound towards the NoMa-Gallaudet University station. At the time of the pull-apart, a rider on the train noted hearing a “loud noise and then everything stopped” as the two parts of the train separated.

Update on today's Red Line event:
The first two cars of the train ended up about 59 feet from the trailing 6 cars of the 6000 Series train.
A tie fire was also apparently put out with a fire extinguisher.
The investigation will focus on a number of areas in coming days & weeks pic.twitter.com/SOaL6jE8KK

— Washington Metrorail Safety Commission (@MetrorailSafety) October 9, 2020

The trailing six cars of the eight-car train detached from the lead pair and came to rest 59 feet away from the front part, according to the Washington Metrorail Safety Commission (WMSC), the independent agency that “oversees and monitors” safety practices on Metro, and is investigating the incident. Photos from the scene appear to show part of the coupler, the component which is used to join one railcar to another, from railcar 6075 coming out of its tube and remaining attached to the coupler from 6079.

Railcar couplers are commonly used within Metro’s rail yards to change the combination of cars that make up trains. If a pair is in need of maintenance, for instance, those cars can be decoupled from a train, or another pair can be coupled up onto the end.

WMSC, which is investigating the pull-apart, will look into various different facets that could have led to the incident, similar to how the National Transportation Safety Board performs its investigations. Items of interest will likely include the history of the railcars, equipment age, condition, and prior maintenance, as well as how that maintenance was performed by Metro personnel. Protocols, and staff adherence to protocol, will likely also be reviewed.

Metro sources, who agreed to spoke Greater Greater Washington under the condition of anonymity, noted two specific components that could play central roles in the investigation into the pull-apart.

What are couplers, and how do they work?

The coupler assembly is what rail operators use to connect one railcar to another. The main “visible” portion of the coupler is the coupler hook at the front which is what is used to “latch” on to the other railcar, but there’s a significant portion of additional machinery behind that. (This description has been significantly simplified for clarity)

Trains can be thought of like an accordion as they move along the tracks, according to one person GGWash spoke to. When a train accelerates, the cars pull apart a little, but the coupler equipment keeps them together, and when the train decelerates the cars press up against each other. A set of large springs, which fits into the buffer tube in the cylinder in the graphic above, helps dampen some of that energy. A set of threads like you would find on a bolt keeps the springs from pulling out of the cylinder.

The two portions of the coupler on railcar 6075 which appear to have separated on Friday are the draft gear buffer - the cylinder seen in the upper-right of the graphic above - and the draft bar, which fits inside. The two halves are secured by a set of interlocking threads and a bolt, while a set of springs inside provide a cushion for when railcars push up against each other or pull apart.

Circled in red on the left, below, is one of four “shear pins” which can be used to help detect a hard coupling when two railcars hit each other too hard. The pins on Metro’s 7000-series railcars are designed to break if they’re pushed with more than 119,823 pounds per foot (533 kN) and will allow the tube’s components to compress more than they otherwise should. When daily inspections are performed, damage to the shear pins would cause a railcar to be taken out of service for a more detailed follow-up.

Shear pins can break if two trains hit each other too hard or potentially if the coupler equipment securing railcars is too loose. The pins are designed to break so more extensive damage isn’t caused to the railcar elsewhere, according to one source, acting as a “safety feature” protecting people and equipment, similar to the way safety glass is designed to reduce the threat of injury if broken.

Circled in orange on the right is the buffer tube bolt (a nut is on the other side, not shown) which helps secure the large spring and rod which typically fit into the tube. An incorrect installation of this bolt was faulted in a 2018 pull-apart on the Silver Line wherein the wrong bolt was installed, the nut on the bolt became cross-threaded, and the assembly loosened allowing two cars to come apart. That incident occurred not at a coupler but between a set of “married pair” cars, two cars which are semi-permanently joined and share sequential car numbers (6000/6001, 6010/6011, etc.), but the resulting pull-apart is similar.

In last week’s pull-apart, even if the correct bolt, washer, and nut were installed, there are other things that could have gone wrong with the installation, says a source. If the bolt wasn’t tightened enough, wasn’t oiled properly, or uncalibrated equipment was used when tightening, they said, the threads on the bar inside the tube could have begun working themselves loose. Visible dirt on the gland nut on the draft bar, the source said, was an indication that they had been exposed to the elements and the connection between the two components may not have been as tight as it should have been.

What happens next

The train involved in Friday’s pull-apart was taken to Brentwood Yard on Metro’s Red Line for further investigation. Metro staff will try to determine what happened, and WMSC will release its report once the investigation concludes.

WMSC could publish a directive if an imminent safety concern is found, but it is likely too early in the investigation to know what, if anything, that could be. For now, Metro continues to run trains on its near-normal pandemic schedule.