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

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

Flat spots r a reality mostly in mixed freight trains, where there r both loads & empties in the train. The empty cars sometimes do not 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.

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 ur stretched u want to keep the slack from running in.If the slack is bunched u want to keep it that way to keep from breaking in two,so u bail the engine brake & 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 u hear on cars in trains going by were caused by cars being dragged around while switching with handbrakes still applied.

Andy is again quite correct. Most all flat spots on cars r caused by moving them with hand brakes applied. l will just add the following.

As far as freight cars r concerned, many r equipped with ''E/L'' brakes, meaning they sense when the car is loaded or empty & 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 DB l (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 & six miles per hour, the Independent Pressure Switch (IPS) nullifies the extended range portion of the dynamic brake so the locomotive wheels wo not slide right at stopping. The setting for the IPS is nominally 12 to 15 psi.

Flat spots r a major problem. Of course they hammer the hell out of the rail, but there is another serious problem caused by sliding wheels, called ''thermal cracks.''

Put the palms of ur hands together, with force, & rub them really fast for a moment or two. They get hot, right away. Now, imagine if ur hands were doing the same thing, but pressing together with 17 tons of force (the area on ur hands is larger than the spot where wheel & 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 & eventually fail if not detected.

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

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

l will 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 & 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 & a good engineer can compensate for that. As a side note, l 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 & never got a flat spot.