"On the high iron, let the big dogs walk"
means the caboose is over the switch and on the mainline so open the throttle all the way on the locomotives

"All black, well stacked, goin' down the track clickity clack"
means the train looked good on the visual roll-by inspection.

"Pull the pin" or "let's pull the pin and roll"
means "uncouple so we can get out of here"

"Highball it out of here."
Proceed at maximum permissible speed

"Double the hill"
means the train is split in half to get up a grade

"We are on the ground!"
means the train has derailed

"Mosey Speed"
means when you approach the limit of your track warrant and have not received a new warrant, you mosey up to the limit prepared to stop.

"Grip"
Trainman's suitcase

"Dead Head"
A railroad employee traveling as a passenger

"Drag"
Describes the movement of a heavy train, such as a coal drag

"Dump the air"
Emergency application of the air brakes causing a train to stop abruptly

"Dog chasing"
A crew change out.

The caller called a dog catch to catch a dog.
Caller – A person whose job it is to call out a train crew.
Dog Catch – A crew sent to relieve a crew that has worked the legal limit.
Catch a dog – Catch a slow moving train.