As you've heard and Alan confirmed, the lounge is EXTREMELY popular during the segment between Denver and Glenwood Springs. The train does stay at the DEN station for a bit, so if you're early in the line to board, you may be able to get on board and have your attendant do the little walk-through all before the crowd of people boarding in Coach get on.

Grab everything you'll need for the day, use the restroom, and then plan to camp out in your seat in the lounge without moving. If you get up from your seat even for two minutes, you WILL find it taken when you return. (Things get a little less competitive after the first couple of hours, once everyone sort of settles in, and if you're sitting next to someone whom you strike up a good rapport with, you can sort of watch each others' seats and discourage others from taking them if either of you has to get up for any reason, but the seats are always up for grabs.)

Have a great trip--this section of track is one of the most beautiful train trips in the world!