When a train goes into Emergency. How do the wheels keep from sliding, and getting flat spots?

Question

When a train goes into Emergency. How do the wheels keep from sliding, and getting flat spots?

Answer 1

Andy is again quite correct.  Most all flat spots on cars are caused by moving them with hand brakes applied.  I'll just add the following...

As far as freight cars are concerned, many are equipped with "E/L" brakes, meaning they sense when the car is loaded or empty and make adjustments in retarding effort accordingly, reducing the probability of sliding wheels.

In addition to "bailing off" the engine (independent) brake, there is additional protection built into locomotives with  dynamic brake.  There is the DBI (Dynamic Brake Interlock), which nullifies independent brake application caused by use of the automatic (train) air-brakes. while in dynamic brake.

When stopping, on locomotives with "extended range" dynamic brake, where the retarding effort remains quite high between as low as ten and six miles per hour, the Independent Pressure Switch (IPS) nullifies the extended range portion of the dynamic brake so the locomotive wheels won't slide right at stopping.  The setting for the IPS is nominally 12 to 15 psi.

Flat spots are a major problem.  Of course they hammer the h**l out of the rail, but there is another serious problem caused by sliding wheels, called "thermal cracks."

Put the palms of your hands together, with force, and rub them really fast for a moment or two.  They get hot, right away.  Now, imagine if your hands were doing the same thing, but pressing together with 17 tons of force (the area on your hands is larger than the spot where wheel and rail meet, which is not much larger than a silver dollar).  They get extremely hot, very quickly, in a very small, concentrated area.  This temperature spike can cause microscopic cracks in the wheel which, over time, will grow, rust and eventually fail if not detected.

With all due respect to Craigory, flat spots on locomotive wheels have never been accepted as part and parcel of normal operation.  If an engineer flats the wheels on the engine during his tour of duty, discipline can and does result.  If you flat the wheels, the locomotive must be removed from service until repaired.  Over and above this considerable repair cost, a $1.2 million dollar piece of equipment that isn't moving any tonnage, sitting awaiting a slot on the "wheel machine" to re-true the wheels, pretty much upsets all concerned.

I can say for myself, as well as every engineer I have ever known, if taking charge of a locomotive consist and discovering flat spots, the dispatcher is immediately informed by radio.  No one wants to take the rap for someone else's crappy train-handling...

Answer 2

In the US, there is quite a difference, depending whether you are talking about freight or passenger trains.

Flat spots are a reality mostly in mixed freight trains, where there are both loads and empties in the train.  The empty cars sometimes don't have enough normal force to maintain rail contact, as the train rumbles along because the loads reduce the overall braking rate of the train.

Many passenger trains utilize sophisticated wheel-slide systems in brake to minimize this problem.  It is most important here, because of the relatively high braking rates of passenger cars, as compared to their freight counterparts.

Locomotives, of late, have also gotten more sophisticated with this issue.  Time was not long ago where flat spots were an accepted consequence, because that was the way that a hogger could "prove" that he used emergency brakes to try to stop to avoid a collision.

Answer 3

The cars weigh enough that they keep rolling because the brake cylinder pressure is limited.But on a locomotive the brake cylinder pressure will keep building.So if an emergency application occurs we bail the engine brakes to keep the wheels from sliding.Bailing is a term for keeping the engine brakes from applying when the train brakes apply.Also to control slack If your stretched you want to keep the slack from running in.If the slack is bunched you want to keep it that way to keep from breaking in two,so you bail the engine brake and still apply them to keep the head end from running out.Engines now have dynamic brake holding feature which lets the dynamic keep working.The dynamic used to quit if the engine went in emergency.

By the way most of those flat spots you hear on cars in trains going by were caused by cars being dragged  around while switching with handbrakes still applied.

Answer 4

I'll simplify things.  There is not enough brake pressure on the wheels to cause them to slide.  Rail cars have what is called a load proportioning valve that reduces brake pressure by 50% on empty cars.  Occaisionally these fail and cause too much pressure on an empty car, but most flat spots occur by moving cars with a handbrake applied in switching operations.  This is also the cause of flat spots on locomotives.  Flat spots also occur on them when "kicking" cars which requires a sudden full brake application on a locomotive.  This usually happens on wet rail and a good engineer can compensate for that.  As a side note, I have operated a locomotive with a faulty air brake computer that would apply full locomotive brakes at 40 mph with 20,000 tons behind it and never got a flat spot.