You got some splaining to do Lucy!?

OK I'll splain it to you.A locomotive braking system is different than a cars.On the locomotive there is something called a j type relay valve.That valve multiplies the air pressure going into the locomotive's brake cylinders.To be honest i don't know which valve they use these days but a common one is 1.7(there are many(well 3 or 4 anyways) different sizes).So for every 10 pounds of air it would put 17 pounds into the cylinder.If your ever on a locomotive and the engineer sets the automatic brake with the engine brakes released(such as for an engine air brake test) watch the independent brake gauge needle.For an example if he sets a 20 pound set with a J 1.7 relay valve the independent brake gauge needle will register 34 pounds.In an emergency brake application if you don't bail the independent brakes the cylinder pressure will keep building til the wheels can lock up and slide(especially as the speed gets lower).In the old days in an emergency application we would fully apply the engine brakes and then bail them to keep the cylinder pressure from building too high if we were in a slack bunched condition(as the speed lowers you still have to partially release the independent brake).Stretched we would just bail the brakes.Now we have dynamic brake holding.That means in an emergency brake application you don't lose your dynamic brake(if your in dynamic brake) when the PC trips.The holding feature is supposed to work for 20 seconds but some seem to work longer before you lose dynamic.We still are supposed to bail the brakes though and them start applying them when needed.I figured my answer would make a few people that are knowledgeable about trains scratch their heads lol.I hope my explanation here helps clear up my previous answer for you.

Edit I forgot to mention you asked about the brakes being connected....The independent brake valve does just control the engine brakes.But the automatic brake valve does actuate the engine brakes also.That's the reason for the bail feature.The only time the automatic brakes don't apply the engine brakes is when your in dynamic braking.We are still supposed to bail the brakes though just in case there is a unit in the consist not equipped with or with a bad order IPS(independent pressure switch).

Edit Thanks Bob I appreciate the housekeeping.And yeah it has been just a few years lol.When i wrote this answer i had been up all night on a wonderful train trip.I just read it again and realized i had forgot to mention the DBI.And i could have swore there was more than 2 different size j valves.I will bow to your expertise however as i know that your much smarter than i am.You must be cause your retired and i'm sitting in a motel room in a little podunk town in New Mexico waiting for a train home.I hope to smarten up myself in about 4 years lol.Again ty for the corrections.

Ten points and a thumbs up to my esteemed colleague above.

I'll just do a little housekeeping.

The J valve is the key. There are two types; the J-1 and the J-1.6 (No harm Andy. I know our trips to the simulator were well over three decades ago). Why two and what do they do?

The air that ultimately goes to the brake cylinders on the engines is carried by the Application and Release Pipe when MU-ing locos. It carries 45 psi. Very old diesel electrics had cast iron brake shoes and had a tendency (pretty much for sure) to make the engine wheels slide if over 45 psi went into the cylinders. But the next and subsequent generation locos started using composition brake shoes that could handle a lot more cylinder pressure, but still had a limit before the wheel slips or slides.

However having a mixture of these units in a consist would create problems without the good ol' J valve. The J-1 sends 45 psi to the cylinders (45 psi x 1 = 45) max and the J-1.6 sends a max of 72 psi to the cylinders (45 x 1.6 = 72).

Why 1.6? 1.618 is a ratio found over and over and over throughout all of nature and is called the "Golden Mean." Google it.

If the train goes into emergency as Andy describes, even more air goes into the brake cylinders exceeding the parameters by a bunch. Ya gotta bail off to prevent this. Often overlooked is the fact that a four unit consist with brake cylinders maxed out can generate enough buff force to jackknife the cars behind the road engine.

The rate at which air flows through the brake pipe ('train-line' is an antiquated misnomer and may give an MOP a heart attack if used in his presence, so by all means, do.) called 'rate of propagation' is 500 feet per second at a service rate and 1,000 feet a second in an emergency application. These figures are rounded off, but very close.

So, if your train is 7,000 feet long, if you make an application of the automatic (train) brakes, it takes 14 seconds for that brake pipe reduction to even get to the last car. If you're doing 60 mph, you've gone a 1/4 mile already and the brakes aren't even really doing anything yet.

Getting a train moving is a relatively simple process. Getting one stopped where you want takes a little practice... that's why you find one system for propulsion on a loco, but four braking systems; automatic air, engine brakes, dynamic brake and hand brake. If you are still moving while cranking on that hand brake you are in the middle of a reeeeealy bad trip...

The DBI (Dynamic Brake Interlock) will keep the loco brakes released in a service application of the brakes while running in dynamic, but there is another little goody at play when stopping the train when in dynamic which is the IPS (Independent Pressure Switch). Nominally set at 15 psi, when coming to a stop and switching over from dynamic to independent, the IPS nullifies the extended range portion of the dynamic brake to prevent the wheels of the engine from sliding.

Good question!

Except for I had to think about it, and now my head hurts. I hope you're happy...

The answer is yes, of course they slide. That is why flat spots develop on the tires. There is no other way that these can develop and I bet that many contributors to this forum will have heard trains passing making the characteristic 'clicking' noise of the flat spots against the rails.

In the UK it is a perennial problem every autumn (fall) and the railway companies always trot out the same excuse - 'leaves on the line' causing sliding. It is probably the single biggest reason for trains being withdrawn from service to enable wheels re-profiling to be performed, sometimes resulting in reduction in scheduled services due to to lack of rolling stock. It can effect locomotive as well as freight and passenger cars, but most folk over here will notice it on diesel and electric multiple unit trains.