Forest Crossen tells the story, with the images here published in his book
The 5th of July  (the 4th was on a Sunday) was a busy day on the Trail, with excursion trains rumbling up and down Four Mile Canon (sic). Every available man was pressed into duty, including some new to the DB&W.
Train No 36 was on its regular trip from Eldora and had made its schedule stop at Salina. Walter Flint of Denver was engineer, Earl Keith, eighteen years old, fireman of No 30.
Conductor C R Phillips gave the highball, and Flint whistled off. The train rolled smartly down the short stretch of the heaviest grade on the railroad. Faster and faster it went, passengers clutching their seats in alarm. Then it happened…
On a curve (Black Swan) the tender suddenly derailed and turned over. The baggage coach ran ahead, tearing off the fireman's side of the cab, shearing steam lines. Instantly high-pressure steam enveloped the cab, billowed back into the baggage car. The mail messenger, George Shull, and the baggage messenger, Harry Cluphf, jumped. Flint was already on the ground.
From the fireman's side came agonizing screams. Flint leaped back into the cab, tried valiantly to find his fireman. The scalding steam drove him back.
The screaming stopped. A low agonizing moan went up. Then silence, terrible silence.
In the following coach, which turned over on its side, General Manager Ford and Conductor Phillips managed to apply first aid to the five slightly injured passengers and calm the others.
The steam finally escaped from the boiler and a hushed silence fell. When the men reached Keith he was dead. Whether he died from the crushing blow or scalding steam no one ever knew.
No man knows exactly why or how the accident happened.page 226
I was curious to see if the site of the Black Swan Mill would be evident, or perhaps built over by a residence as so many other sites have been. Not a problem--it was clearly identified by a sign.
From there it was just meters away to the curve.
Exploring the site, I contemplated the events of that tragic day. Cars came flashing by, workers on their way to Boulder full of their own anticipations on a busy Monday morning.