Re: [Phys-L] train stopping distance

[John Denker <jsd@av8n.com>]

On 06/23/2016 05:10 PM, LaMontagne, Bob wrote:

> Even when the wheels are locked, it takes over a mile to stop a
> moving train on flat ground.

1) They don't lock the wheels (except maybe in cowboy movies).
 There's no upside to doing so, and a tremendous downside.

2) I looked up some data and did the arithmetic:

				emergency
	speed			stopping distance			accel						
car:	55	mph		200	feet								
	24.6	m/s		61.0	m		4.96	m/s^2		0.506	 Gee		https://www.minnesotasafetycouncil.org/facts/factsheet.cfm?qs=858251BECECF1976F908D7D68B570E85
													
truck:	55	mph		300	feet								
	24.6	m/s		91.4	m		3.31	m/s^2		0.337	 Gee		ibid
													
passenger	80	mph		1	mile								
train:	35.8	m/s		1609	m		0.40	m/s^2		0.041	 Gee		ibid
													
freight	55	mph		1	mile								
train:	24.6	m/s		1609	m		0.19	m/s^2		0.019	 Gee		ibid
													
coal	50	mph		0.75	mile								
train:	22.4	m/s		1207	m		0.21	m/s^2		0.021	 Gee		http://cs.trains.com/trn/f/111/t/72641.aspx
													
manifest		worse than the above											
train:													ibid
													
suburban 70	mph		0.5	mile								
train:	31.3	m/s		805	m		0.61	m/s^2		0.062	 Gee		ibid


The coefficient of friction for steel-on-steel is on the order of 0.5,
so when you see an acceleration of 0.02 Gee you know they're not trying
very hard.




On 06/24/2016 06:15 AM, Bill Norwood wrote:

> > If the train could muster enough braking friction to stop in 1/10 the
> > distance would that buckle the track?


Innnnteresting question.  The short answer is that this is definitely
something worth worrying about.  Quote:  
  "Slack action, heavy dynamic braking and emergency brake
  applications can trigger a buckle."
That's from:
  "Track Buckling Research"
  https://www.volpe.dot.gov/infrastructure-systems-and-technology/structures-and-dynamics/track-buckling-research
  http://ntl.bts.gov/lib/50000/50700/50710/Track_Buckling_Research_2003.pdf

You can also try
  https://www.google.com/search?q=%22Rail+Longitudinal+Force%22

In any case, a factor of 10 is so far beyond previous experience that
I reckon all bets are off.  Imagine trying to test the proposition.
The tests could be amazingly expensive.  Even if you found that track
section X could withstand the force, it's not clear you could trust
section Y.  You would need to worry about force per unit length as
well as total force.  That is to say, a long heavy train is different
from a short heavy train.

Typical fasteners (for holding rails to the sleepers) look to me
like they're designed to constrain movement in every direction
/except/ longitudinal.  There must be ways of transferring 
longitudinal momentum to/from the earth, but the details don't
seem particularly obvious.