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  • Braking / deceleration distance or time of WMATA trains

  • Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.
Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.

Moderators: mtuandrew, therock, Robert Paniagua

 #1413322  by MCL1981
 
I just happened to notice a statistic on the wiki page that lists WMATA trains deceleration rate as about 3 mph per second in emergency. That would make the time to go from 55mph to a stop in emergency about 17 seconds. That couldn't possibly be the case. I've never had the displeasure of being on a Metro train when it goes into emergency at 55mph. But I can recall plenty of what seemed like full service braking from a high speed. And it seems like it was single digit seconds. I've been on a loaded MARC train with 6 cars in emergency from 55mph, it was single digit seconds I think, with a lot more mass to stop.

Anyone have any data on actual stopping distance or time?
 #1413346  by dcmike
 
Emergency brake rate for all WMATA fleets is -3.2mphps (miles per hour per second) and wheel slide protection is inhibited.

Full service brake, or B4, is -2.2mphps.

Max service brake, or B5, is -3.0mphps. Unfortunately the preferred notch for all brake applications by operators running in manual. The ATO system almost never uses this rate, relying instead on a combination of notches ranging from COAST to B4.
 #1413351  by dcmike
 
In practice, it's not unusual for the trains to overshoot that rate by a little bit. I've seen rates between -3.6 and -3.8 on post-incident analysis reports following station overruns or the like pretty frequently. Emergency brake applications are fully friction brake (no electrodynamics) operating in open-loop mode; that is, no feedback inputs such as speed sensors or pressure transducers are being monitored that would permit the system to make corrections.
 #1413369  by farecard
 
dcmike wrote:In practice, it's not unusual for the trains to overshoot that rate by a little bit. I've seen rates between -3.6 and -3.8 on post-incident analysis reports following station overruns or the like pretty frequently. Emergency brake applications are fully friction brake (no electrodynamics) operating in open-loop mode; that is, no feedback inputs such as speed sensors or pressure transducers are being monitored that would permit the system to make corrections.
It's always amazed me that locked-up [steel railroad] wheels stop in less distance than rolling ones. But I've read that is the case....