Braking / deceleration distance or time of WMATA trains

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?

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.

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.

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.

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....