Denver's Union Station has no place to check bags, at least as of August, 2024. They used to. The station has a very nice hotel called the Crawford, which used to check bags for a fee. They no longer do. The station also services Amtrak, and Amtrak used to have lockers, but they no longer do. I have no idea why, but it's easy to guess.
I had checked out of my hotel in Boulder, taken the (very nice) Flatiron Flyer bus back to Denver, and now I had time to kill before my evening train back to Chicago. Where could I park my bags, so I could walk around and explore, unfettered? I asked at the front desk of the Crawford Hotel and the nice lady there gave me a card for LUGDEN. LUGDEN is....a van. A van parked on Wynkoop Street, by 17th, right across from Union Station. They'll let you check luggage for a fee (at the time of writing, $3/hour or $10/day). I was able to check both of my bags successfully, head to Meow Wolf, followed by lunch and wandering, then swing back and pick up my bags in time to grab a pre-train cocktail at the Terminal Bar.
So, I'm sharing this here for those who, like me, searched online and found information lacking. You'll find old message board posts that suggest that Amtrak and the Crawford will check bags for a fee, but then neither Amtrak or the Crawford's website say this any more, and when I asked at the hotel, they weren't storing bags for non-guests. So, anything you see online suggesting otherwise is probably out of date.
Need to store luggage at Denver's Union Station? You'll want to look for LUGDEN. Find their website here, get a card from the front desk of the Crawford Hotel, or look for them on Wynkoop Street.
While it may seem to be a bit of an odd setup, everything worked fine, the process was easy, the person manning the van was professional, and they do take credit cards. I had no issues at all and would recommend them to others.