Train brakes and wet tracks

[gotpasswords]

How much effect does wet rails have on the effectiveness of train brakes?

It’s been raining here, and the commuter trains are running at reduced speeds for safety considerations. Do they really need to?

Some numbers…the weights come from the transit agency, and my estimation of contact patch comes from mumerous cites of being about the size of a dime.

I’m on a ten-car train. Each car weighs in at right around 62,000 pounds, and at commute times, the “crush load” of humans weighs in at about 20,000 pounds per car, making a total of about 82,000 pounds per car, or 820,000 pounds for the whole train. (Now you know why it’s called Heavy Rail!)

Obviously, the braking systems - a combination of dynamic regnerative “motor” brakes and plain ol’ friction brakes has to be up to the task of clamping down on all that weight at speeds of up to 70 miles per hour through a contact patch of about a half square inch per wheel, or roughly 40 square inches of steel wheel to steel rail contact for the entire train. While this does seem a monumental task, trains do it thousands of times a day without incident.

Are train brakes, or the coefficient of steel on steel friction really so marginal that they need to slow down when the rails are wet?

[Q.E.D]

@gotpasswords

Are train brakes, or the coefficient of steel on steel friction really so marginal that they need to slow down when the rails are wet?

I think it’s more an issue of the wheels getting wet and reducing the friction between them and the brake shoes than the friction between wheels and track.

[Una_Persson]

According to the 70 year-old rail expert guy I hired who’s sitting across the row from me, the rain does have an impact on the braking force you can apply, due to track conditions. However, he is short on citations, and very grumpy. So I hit Google to try to find some quick examples. They may not be the best.

From: http://faculty.ist.unomaha.edu/winter/publications/2001/Bay_Area_Rapid_Transit_District_Advanced_Automated_Train_Control_System_Case_Study_Description.pdf

The maximum brake rate may not be achievable in practice. If wheel slippage is detected, the controls to that set of wheels reduces the braking force so that wheel adhesion can be regained. Wheel adhesion and slippage limits, not the capabilities of the braking system, often determine how fast a train can stop. The worst case profile to be discussed below assumes only decelerations of 1.2 mphps on wet track and 1.6 mphps on dry track, although these are very pessimistic bounds.

(Adhesion can also limit acceleration performance, although this is not critical for safety. Note also that when the wheels are slipping, tachometer data will not be representative of real speeds.

The minimum brake rate is 0.45 mphps.

“mphps” is “mph per second” speed reduction. So from what they’re saying, about a 25% decrease on wet rails.

[Una_Persson]

Some other links - see here, page 58 (describes wheel lockup similar to a car): http://www.jrtr.net/jrtr20/pdf/F52_Tech.pdf

See also here: http://www.railway-technical.com/brake1.shtml

Slip/Slide Control

A critical feature of the railway environment is the interface between wheel and rail. This interface is dependent upon the adhesion between the steel surface of the wheel tread and the steel surface of the rail head. The relationship is defined as the coefficient of friction. On a dry day is this is about 0.3, on a wet day 0.2 with clean rails. A figure of 0.1 is allowed for normal braking and 50% of that added as a safety margin to prevent overrunning. Values under 0.05 will occur in conditions where the rail head is contaminated by leaves or ice.

So this seems to say a 33% reduction (0.3 -> 0.2) in braking force is possible with wet rails.

My IRL source, Grumpy McGrumpington, simply yelled at me “It takes twice as long, who wants to know?”

[Superfluous_Parentheses]

As a small additional data point: the newer trams in Amsterdam can dump sand onto the tracks wile/prior to braking. You can even see the sand level through a few small vertical windows next to some of the seats.

I asked a tram driver about them, and he said that was used when the track was wet or snowy. I’m not sure that trams even use the same mechanism to break. Still, quite interesting.

[DJ_Motorbike]

@Superfluous Parentheses

As a small additional data point: the newer trams in Amsterdam can dump sand onto the tracks wile/prior to braking. You can even see the sand level through a few small vertical windows next to some of the seats.

I asked a tram driver about them, and he said that was used when the track was wet or snowy. I’m not sure that trams even use the same mechanism to break. Still, quite interesting.

Sanders have been common on trains since the steam era.

[kanicbird]

Another issue is that any oil that is on the rail, that probably sunk into small imperfections in the metal will rise up and be distributed over the rail if water is on the rail. Much like driving on the road during the start of a rain shower can rise oil from the road.

[Broomstick]

My only source of information is riding on trains almost daily for 25 years.

Yes, wet conditions can and do affect braking. I have been on trains that have simply skidded past station stops, even while traveling at low speeds, due to slick tracks. I’ve also seen trains slide into stations with their wheels locked motionless but still going because of slick conditions. Sanding can help but there are times, particularly when the rain is freezing, that even that can’t help and very slow speeds and allowance for greatly increased stopping distances are required.

[gotpasswords]

Grumpy Gus notwithstanding, I’m surprised that there’s that much difference when each point of contact is bearing about 20,000 pounds per square inch. I’d have expected that much pressure to just vaporize any nearby water molecules, not to mention squash any ice or leaves into oblivion, but that’s obviously not the case.

[Una_Persson]

@gotpasswords

I’d have expected that much pressure to just vaporize any nearby water molecules, not to mention squash any ice or leaves into oblivion, but that’s obviously not the case.

To expand on this, you might want to search on some of the problems the UK has had with leaf slime on the rails. They changed to a new treatment a few years back that helped, but leaves on the rails are still a hazard.