"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.

From George Martin:

My Grampaw, who was born in 1893, was a railway worker for a time. He taught me a song that I have lost the written lyrics to, but think I know most of them. I couldn't find these slang terms on your site, so I thought you might like them. My Grampaw worked in Kentucky, not Alaska, but I imagine the terms were the same.
Do you know this song?

"Giro Giro"

"Giro giro get on the gyro
gyro gyro fly the rod
with a rapstack yellow bug
yellow bug a fumidoodle
rapstack barney catch akimbo"

Which means:

Giro: a person who works the handcart
Gyro: The handcart (That odd cart that goes on the rails and uses a sort of ratcheted hand pump to turn the wheels.)
Fly the rod: Go down the rails
Rapstack: High pressure in a steam engine causes the "stack" or equivalent of a chimney, to rattle and rap. Rapstack means full steam, or as fast as you can go.
Yellowbug: a yellow railroad lantern used as a signal light
Fumidoodle: swing the signal lantern in a particular patter as a warning.
Barney catch: An iron arm that swings out over the track to allow mailbags to be picked up "on the fly", which still means when going by.
Akimbo: sticking out to the side.

So the song means:
"Hey yard worker, get on the handcart and go down the line as fast as you can to signal the train with a warning to slow down. We've got a mail bag hanging out to be picked up as you go by."

He taught me another song, called "The Gandy Dancer's Song", about Gandy Dancers, but I don't remember the lyrics.

He told me Gandy was a company that made railroad tools used to position rails. One such tool was like a big lever or pry bar, and another like an ice-pincer or ice-tong, but made to grip a rail. Gandy Dancers were men who carried a rail by straddling it and used a Gandy to pick it up, then they waddled in-step together to move the rail. The waddling was called the Gandy dance.

I believe the workers were divided into section workers, or field workers, those that worked on the train, those that worked in the station, and those that worked in the yard. Gandy Dancers were section workers.