Hello Stephen,

> Valentin Brueckel wrote:
> 
>> R.C. Payne  schrieb/wrote:
>> 
>>> Though ICEs (in common with higher speed German trains) have some
>>> form of magnetic track brakes.
>>> 
>> They have them for a reason. And no, magnetic track brakes are not
>> limited to high speed trains.
>> 
> Certainly not.  DART's LRVs (top speed 67mph) have track brakes.  I've
> seen them used once, when a train was slipping on wet rails and about
> to overshoot a platform.  There was a loud crack (almost like thunder)
> and the train went from 20mph to stopped in under a second -- very
> impressive.  Unfortunately, I've never seen such brakes on FRA trains,
> where they're more desperately needed.


If they really are that strong, managing a deceleration of over 1g,
then you probably don't want to use them when overshooting a
platform, as a sudden, unexpected 1g acceleration seems rather
risky to the passengers.  It is better to miss a stop.


Documentation on magnetic track brakes I have seen elsewhere
gives good deceleration rates, but still much lower than 1g.
For example http://www.calgarytransit.com/html/technical_information.html
lists Calgary's LRV's maximum deceleration as 3.0m/s^2,
or 0.3g.  (Serrvice deceleration is 1.3m/s^2, or 0.13g.)


Greg Gritton

Greg Gritton wrote:
> Hello Stephen,
>> Valentin Brueckel wrote:
>>> They have them for a reason. And no, magnetic track brakes are not
>>> limited to high speed trains.
>>>
>> Certainly not.  DART's LRVs (top speed 67mph) have track brakes.  I've
>> seen them used once, when a train was slipping on wet rails and about
>> to overshoot a platform.  There was a loud crack (almost like thunder)
>> and the train went from 20mph to stopped in under a second -- very
>> impressive.  Unfortunately, I've never seen such brakes on FRA trains,
>> where they're more desperately needed.
> 
> If they really are that strong, managing a deceleration of over 1g,
> then you probably don't want to use them when overshooting a
> platform, as a sudden, unexpected 1g acceleration seems rather
> risky to the passengers.  It is better to miss a stop.

At the end of a platform was a traffic light (not a standard active 
grade crossing), which was at the time giving a green light to cross 
traffic.  Avoiding an accident there, which likely would have killed 
people, was the better choice even if it meant banging the train's 
passengers around a bit.

My ability to estimate speed isn't that great, so perhaps 20mph is on 
the high side; it seemed to be going faster than 10mph, and I know 
trains there have a speed limit of 30mph, so I split the difference and 
figured it was close enough for an anecdote.  I've never found any 
official stats on the deceleration rate for those LRVs' track brakes, 
just 1.3m/s^2 for the regular ones.

S

In article <t3tAk.644$x%.361@nlpi070.nbdc.sbc.com>,
 Stephen Sprunk  wrote:

> Greg Gritton wrote:
> > Hello Stephen,
> >> Valentin Brueckel wrote:
> >>> They have them for a reason. And no, magnetic track brakes are not
> >>> limited to high speed trains.
> >>>
> >> Certainly not.  DART's LRVs (top speed 67mph) have track brakes.  I've
> >> seen them used once, when a train was slipping on wet rails and about
> >> to overshoot a platform.  There was a loud crack (almost like thunder)
> >> and the train went from 20mph to stopped in under a second -- very
> >> impressive.  Unfortunately, I've never seen such brakes on FRA trains,
> >> where they're more desperately needed.
> > 
> > If they really are that strong, managing a deceleration of over 1g,
> > then you probably don't want to use them when overshooting a
> > platform, as a sudden, unexpected 1g acceleration seems rather
> > risky to the passengers.  It is better to miss a stop.
> 
> At the end of a platform was a traffic light (not a standard active 
> grade crossing), which was at the time giving a green light to cross 
> traffic.  Avoiding an accident there, which likely would have killed 
> people, was the better choice even if it meant banging the train's 
> passengers around a bit.
> 
> My ability to estimate speed isn't that great, so perhaps 20mph is on 
> the high side; it seemed to be going faster than 10mph, and I know 
> trains there have a speed limit of 30mph, so I split the difference and 
> figured it was close enough for an anecdote.  I've never found any 
> official stats on the deceleration rate for those LRVs' track brakes, 
> just 1.3m/s^2 for the regular ones.

20mph is 8.9 ms^-1.  Going from that to stopped in 1 second would be 8.9 
ms^-2, or nearly 1g (9.81 ms^-2).  At the 3 ms^-2 that other people have 
quoted for emergency brakes makes for 3 seconds or so from 20 mph or 1 
second from just under 7 mph.

Sam