Denver International Airport: Take RTD’s A Line commuter train straight to Denver Union Station: about 37 minutes. It is the sane default for downtown/LoDo, roughly $10 one way; do not pay for a taxi or rideshare unless your hotel is far from rail or you are splitting the cost with several people. DEN is 18 miles east-northeast of downtown, so road trips can be slow and expensive at peak times.
Denver Union Station: Amtrak’s daily California Zephyr arrives here, as do RTD commuter rail, light rail and buses. The station is genuinely central—walk to LoDo and downtown, or use it as the transfer point for the A Line to DEN. The Zephyr’s westbound morning run has outstanding mountain scenery, but long-distance delays are common; do not schedule a flight or nonrefundable event tightly after arrival.