The Origins of Metro’s 26 Trains Per Hour Limit
Why can’t Metrorail reliably operate more than 26 trains per hour per direction? A 2001 study defined the basis for determining this constraint.
Although it has been known for years, the July 2014 opening of the Silver Line and corresponding reduction of rush hour Blue Line service highlighted that Metro cannot schedule more than 26 trains per hour (or a train every 2-2.5 minutes) across any point in its rail network. Though it’s been discussed many times through the years, let’s explain this limit in detail now.
The components of Metrorail’s minimum sustainable headway are: minimum train separation (about 70 seconds), governing dwell time (about 40 seconds), and what’s called an operating margin (about 25 seconds). Train separation is determined by primarily by the vehicle’s ability to accelerate quickly up to its maximum speed and brake safely to a stop, the train’s length, and the configuration of stations, and tracks. Dwell times are determined primarily by the number and width of railcar door openings and the passenger volumes at major stations on each line. The operating margin is an amount of time between successive trains that is inserted into a timetable to accommodate minor delays without significantly impacting following trains. The total of each of these values (70+40+25) yields a minimum sustained headway of approximately 135 seconds between trains, which equals about 26 trains per hour.
This limit was put to the test in 2000 where, in response to the “Orange Crush”, Metro scheduled 29 trains per hour (19 Orange, 10 Blue) through the Rosslyn portal. However the aggressive 29 trains per hour (125 second headways) schedule was rarely achieved even with a mix of shorter 4- and 6-car trains, which tend to favor close headway operation. This was primarily due to tight operating margins and unattainable dwell times, scheduled at 12 seconds outside, and 19-23 seconds inside the core (far lower than the study’s recommended 40 seconds). Metro’s train control system can accommodate close headway operations, so in certain instances one can observe successive trains less than 135 seconds apart. Nevertheless this typically occurs in certain core track segments and cannot reliably be sustained over an entire rush period, day after day across the entire network.
The study (see above graphic) revealed that lower Metrorail operating margins corresponded to reduced on-time performance. With low schedule margins, instead of being able to absorb minor delays and quickly recover, small train delays or dwell time variations propagated to multiple following trains. As this margin approaches zero the number of following trains that will be delayed by an incident ahead increases exponentially. In the simplest terms, as train throughput approaches its upper limits, service reliability tends to drop.
In recent years, reliably achieving 26 trains per hour through Rosslyn has at times been difficult, which may argue for even more emphasis on preserving schedule reliability over attempting to schedule trains more closely together. This constraint also underpins efforts to plan for additional capacity, particularly at Rosslyn. In a future post, we’ll talk about how rail operating conditions have changed in recent years and whether there are strategies available to mitigate this capacity limitation.