Braking / deceleration distance or time of WMATA trains

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Braking / deceleration distance or time of WMATA trains

Postby MCL1981 » Tue Dec 20, 2016 12:31 pm

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?
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Re: Braking / deceleration distance or time of WMATA trains

Postby dcmike » Tue Dec 20, 2016 3:44 pm

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.
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Re: Braking / deceleration distance or time of WMATA trains

Postby MCL1981 » Tue Dec 20, 2016 3:51 pm

So those are the specifications. But what actually happens? With wheel slide inhibited, does it slow down faster than 3.2mphps? That just seems like a REALLY long time compared to what I've experienced in reality.
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Re: Braking / deceleration distance or time of WMATA trains

Postby dcmike » Tue Dec 20, 2016 4:15 pm

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.
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Re: Braking / deceleration distance or time of WMATA trains

Postby farecard » Tue Dec 20, 2016 6:03 pm

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....
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