Denver International (DEN): A Traveler's Guide
Denver International Airport (DEN) is famously big — its site is larger than the city of Boston — with a single central Jeppesen Terminal linked by an underground train to three concourses (A, B, and C). Concourse A also has a pedestrian bridge if the train is down, but plan for a genuine walk-and-ride between check-in and your gate, especially to Concourses B and C.
Immigration and security
International arrivals clear US Customs and Border Protection at the Jeppesen Terminal before continuing to any domestic connection; automated kiosks and Mobile Passport Control speed this up for eligible travelers. TSA security lines are generally manageable outside of peak holiday travel and Colorado's ski season surges, but DEN's size means the walk from checkpoint to gate can eat 15–20 minutes on its own — arrive the standard two hours before domestic flights and three before international ones, and lean toward the longer end during winter storms, which regularly disrupt Denver's schedule.
Getting into the city
The University of Colorado A-Line commuter rail is the easiest way downtown: trains leave from the airport's own station every 15 minutes at peak times (every 30 minutes off-peak) and reach Denver Union Station in about 37 minutes for a flat $10 fare each way — from Union Station you can transfer to light rail or buses across the metro. Taxis and rideshare cover the roughly 25-mile drive to downtown in 40–60 minutes depending on traffic, typically running $75–90 for rideshare and somewhat more for a taxi. Rental cars are plentiful at the airport if you're headed to the mountains rather than downtown — Red Rocks and the ski areas are both easier reached by car than by transit.
Practical tips
Denver sits a mile above sea level and the airport itself is out on the high plains, so it's worth carrying water and going easy on alcohol on arrival if you're not used to the altitude, particularly if you're heading straight up into the mountains. Wi-Fi (network "DEN Airport Free WiFi") is free, fast and genuinely unlimited with no login page, which makes the terminal a decent place to ride out a weather delay. If your connection is tight, note that DEN's concourse-to-concourse train rides add real time on top of walking — check your connection window against the airport's own guidance rather than just the airline's minimum, especially in winter when de-icing delays are common.