I was recently talking to a trainmaster with over 30 years experience, we were discussing the GN. He brought up a story about how somehow a GN passenger train got routed (due to a signal failure) over a diamond. But the twist is that there was a long slow freight still going across. As soon as the engineer saw this, he had to make a decision in 2-3 seconds. It was whether to shoot the works or make a maximum service application. He made the maximum service application and stopped a few feet from the train. If he had gone into emergency the wheels would have locked up and slid, therefore causing the train to go a little bit farther, therefore causing a very deadly accident.
So my question is, If going into emergency causes you to slide and go farther than a Maximum Service Application, whats the point of going into emergency?
The wheels do not necessarily lock up and slide in an emergency application. I was riding a suburban train on Metra’s Southwest line a few years back that went into emergency at the time the train hit a semi-trailer in the grade crossing at Cicero Ave. None of the wheels slid.
Im betting the Engineer knows the rails and mentally cast the dice for a full service application for best stopping in that moment in time. If he blew it down coming up on a busy diamond it will take him a few minutes to pump it all back up and settle the train again. That might not have been acceptable to the current schedule of trains bearing down onto that diamond later in the hour.
As a former trucker there are several different methods of stopping in a hurry and only a few seconds to decide how to do it. In the pre-ABS days your choice of braking method will determine the result once the physics of momentum versus braking horsepower/availible traction and taking into account road conditions.
Since I had ABS, I have been surprised by “Pop-up” situations involving other people in bad weather. Those usually result in the toss of my Anchor with the resulting flats or bad tires due to meltdown of the road surface. The big difference was the technology allowed me to stop in time. I can count 4 events of full-stop and about 8 lives saved total.
The rest of the discussion regarding availible braking for the distance is moot. The engineer only had a few seconds to decide and do it. Everything else will either be a accident or a successful stop just have to put it all to the rubber and wait to learn if your choice is right or wrong.
You probably are going to be keel-hauled by the supervisors and some big suits that will converge on the scene or at your ternimal to grill you with possible penalties once they determine the “Trigger” event that allowed the incident to be set up. The SIgnal Failure must have been a root cause and it better indeed be a failed signal!
Well I suppose hes not a real trainmaster, more of a General Manager of Operations, and he knows everything there is to know. He knows locomotives inside and out, and is the smoothest engineer ive ridden with.
So what exactly makes the wheels lock up?
btw csxengineer98 it was nice seeing you around here again.
I’ll second that idea. Dale (nanaimo73), sometime back refered to you as “one of the most human posters on our forum”-a high compliment, that I agree with 100% . Us non-railroaders appreciate your down to earth perspective on railroading. Thanks.
The wheels lock up because the brakes push against the wheels with enough force that the frictional force between the brakes and the wheels is enough to stop the wheels from turning but the frictional force between the rails and the wheels is not enough to stop the train.
wouldnt a light loaded train be more likely to “Lock Up” than a heavy loaded train?
If so, and the engineer knew his train, he may have been more than “Lucky”. Sounds like someone who knew his job pretty well…the ultimate judgment that he was correct is that the collision was avoided.
I’m not sure of the braking charcteristics of North American passenger trains but where I live (Britain) most passenger trains have a 3 step braking system with a 4th Emergency position.
The wheels will often lock up due to the railhead conditions causing the train to lose grip with the rail combined with a heavy brake application. (Rather like braking a car hard on a wet road). On our trains if the rail is very slippery the wheels can lock up and the train can slide even in ‘Step 1’ (the lightest braking position) or alternatively in good conditions the train can be brought to a stand in Emergency without the wheels locking. Another factor is the brake forces aren’t always exactly the same on each train in each braking position. For example Step 1 on one train may provide a larger brake force than on another train of the exact same type. They are all roughly similar though. For this reason Drivers here are required to undertake a ‘running brake test’ to check the effectiveness or the brakes. If they are bad the train will be removed from service but there will always be trains with slightly better or worse brakes than others for any number of reasons yet still be within a safe working capacity. The Running brake test also allows the driver to get a feel for the railhead conditions and in weather where it is likely to be slippy they are advised to regularly apply the brakes to ‘test’ the railhead and see if the wheels lock more easily than they would in perfect conditions. In bad conditions ‘earlier and lighter’ is the advice for braking. This is mainly to reduce the chance of overrunning a station or a stop signal.
Normally when the train slides the driver would usually release and re-apply the brake in order to get the wheels turning again and hopefully re-apply brake force without the wheels locking.
So it becomes a physics equation. If the train is loaded, the wheels are less likely to lock because the downward force against the rail is greater. But an empty train will stop quicker than a loaded train even if the wheels lock because it has less forward momentum to begin with.
You do not need as much force between the wheels and the rails for a lighter train to stop it in the same time. In that respect the wheels will be less likely to slip.
a = Rolling stock accelaration
B = Force brakes push against wheels
FB = Frictional force between brakes and wheel
FR = Frictional force between wheel and rail
g = Acceleration due to gravity
m = Mass of rolling stock
I = Moment of Inertia about center of wheel
r = Radius of wheel
u1 = Coefficient of friction between wheel and rail
u2 = Coefficient of friction between wheel and brake
W = Weight of rolling stock (W=mg)
α = Angular acceleration of wheel
Summing the horizontal forces, assuming the train is on a level surface, we find that FR=ma. Since FR=u1W, u1W=ma. Summing the moments about the center of the wheel yields FR-FB=Iα. This is equivalent to u1W-u2B=Iα. The accerlation at the wheel rim is a=rα. Therefore, (u1W-u2B)/r=Ia or a=(u1W-u2B)r/I.
a=(u1W-u2B)r/I (Equation 1)
a=u1W/m (Equation 2)
The acceleration in Equation 1 is the maximum acceleration the brakes can cause. The a
Usually your emptys will lock up and slide with an emergency application or tight handbrake. Loads will once in a while but usually they will just make a loud noise and you can smell the brakes burning as well as see smoke and sparks.
Reacts by applying brakes (give 1/2 second or so for the pucker factor to wear off)
Heavy trucks air systems can apply full braking horsepower within a second or two to the wheels. It will be up to the tires to apply that force against the highway and then the final result will be determined by the availible traction, braking horsepower and load weight. (Mass, momentum etc)
I think I would be able to execute a stop with a heavy truck at 40 ton on dry pavement from 70 to 0 in roughly 450 feet and about 8 seconds or less but it’s gonna be a hell of a stop.
The opposite scenario is Ice. Make it a totally empty truck at about 27,000 pounds tare at 70 mph and the stopping distances are over 1/4 mile and time is so long as to have plenty of time to experience the accident, and still be working on the stop.
On a serious note - stopping an empty on dry pavement may actually take longer, since the coefficient of friction will be lower. Throwing in the difference in forward momentum/weight may balance the two out.
The full load on ice, however, will have a fairly short stopping distance, as the coefficient of friction between the side of the truck and the dirt in the median will be quite high…
oh hell…now your going to make my head swell up to the size it once was the day after i got my promtion to engineer…lol
its good to be back… still dont spend as much time online at the computer as i use to to keep an eye on the treads…alot of major personl life changed have taken place in the past few months for me…some good…some not so good… have taken up alot of my free time…but ill do my best to make my “fans” happy…lol
I thought I had a 10 gallon hat and a very big head when I was promoted Trainer some years ago… whew a salary and everything.
Regarding the empty on dry pavement scenario… I’ll try it once. Let’s see a Mack R model two screw model with a 40’ Gooseneck chassis (Container frame with no container) and brakes that dont know any better made for some jackknives or bad hopping that actually required one to take the foot off the service pedal a tad, something not a natural action when a pending accident is happening in a few seconds.
The good news is braking systems get smarter and compenstate somewhat for the empty weights on dry. The worst condition is a very light aluminum flatbed on greasy slick rain/oil mix as found in Louisiana and other places where the concrete interstate is worn smooth as a glass plate. Ive seen stops where I have one end on the other side of the surprised and scared car on my front end.
I hate to think about what the sensation is like w