jaycoffman Loc: San Diego

Window reflection--I don't know the train you're taking but some of the trains I've been on have open areas in the space between the cars. Check that out--in June it will still be cold up there but not as cold as other seasons. Or it may have an observation car which generally the glass isn't as reflective as regular train windows so check on that before you go.

As far as lens--what do you like to shoot and how? If you know that it should answer your question. For example, I'm not big on wide landscapes but I like very specific parts of landscapes as well as flora and fauna. So I tend to go with more zoom. If I were choosing from your equipment I'd probably take the Sigma 120-400 as more magnification and more weight would only interfere. But, I just used me as an example--you need to think what types of shots you'd like to get the most of and remember you'll be moving a lot of the time.

Also, watch for curves--they provide some of the best train shots you can get when you can see the engine going around a beautiful curve. No matter what you're going on one of the most beautiful fried anywhere so you should enjoy it.

DJ Mills Loc: Idaho

This trip is on my bucket list, but I'll never get there. Please post lots of photos for us (me) to enjoy.
DJM

xt2 Loc: British Columbia, Canada

The train ride is wonderful! There is nothing special regarding the lens you bring though. One wide and one long. Simply bring you favourites...

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

The 18-200 and 12-24 are DX lenses so leave those at home. Remember that you will be shooting from a moving train through glass, unless you shoot between the cars (and most trains will not let you do that). Sometimes the tint on that glass will change you color balance, call the train line and ask about their glass. DO NOT TOUCH ANY TRAIN PART WHILE SHOOTING, NOT A GOOD RESULT). If you take shots from a moving train you minimum shutter speed should be at least 1/1500 sec. or higher. And, if you take the long lenses, shoot even faster because the shake of the train will be magnified the further you zoom out with those long lenses (1/2500 sec. at 600 would be minimal and even at that you could end up with some bummers). If it was me I would use the 24-120 on the train. The Sigma 150-600 is a nice all around for mountain ranges but honestly that lens would be hard to shoot from a moving train. The Sigma 200-400 might be a better choice. Have a great trip and post some shots for us to see.

Rashid Abdu Loc: Ohio

I took a train from Alliance Ohio, to Denver. I used Nikon 20mm f2.8 lens with my Nikon D800E, with aperture priority, and got great shots from moving train. Yes, I shot through the glass from the observation car.