I was watching a rr film of locomotives pulling an 80 car line of coal cars, and it said that the dispatcher let it use the #2 track because of a gentiler grade and not risk it PULLING A KNUKLE. never heard of that term, what does it mean.?

You can break a coupler knuckle by over-stressing it, i.e. too hard a pull with a given train weight or running out the slack hard or a combo of both. Sometimes they just break from long hard use or an internal flaw lets loose. All things considered, busting a knuckle doesn't happen very often. When it does, you sit and wait for a carman to come replace it or, less likely, if they can't and you have a spare, you lug one back and change it yourself. Not fun. Those buggers are heavy.

Eric, thank you so much for the explanation of pulling a Kunkle. that is why that the dispatcher let the coal drag use track #2 because of the less of a grade lessening the amount of force applied to the knuckles. it as you said must have been a herculean task for the crew as you said lug one of those heavy monsters back and replace it. being that it could happen on a grade what kept the cars at / behind the break from be comming a run a way consist on cars. just curious.

Air brakes are fail-safe. If the train comes apart, the air hoses separate dumping all the air in the train line and everything goes into an emergency brake application. So even the cars that are no longer connected will apply their brakes. Brakes apply according to a reduction in air pressure in the train line. The bigger the reduction, the harder the brakes apply. That's why before a train can move, you have to wait until the entire train is pressurized and you can release the brakes. There's actually more to it in managing your air in train handling but that's it in simple terms.

Eric, again thank you man for the detailed explanation of the air brakes locking up automatically when a knuckle breaks and sets the whole detached consist in to automatic lock down, saving a run away a great built in safety feature. I am getting a history lesson on great subjects. thanks guys.