What we know today as the Moffat Route started back in 1902 as the Denver, Northwestern & Pacific, David Moffat's dream made real about a railroad straight west from Denver to Salt Lake City. The line was intended to follow a steady 2% grade up through numerous tunnels in the Flatirons and along South Boulder Creek until reaching the main range, where it would eventually tunnel beneath the highest mountains and work west. The line would then follow the Grand (now Colorado) River westward through Hot Sulphur Springs, Kremmling, and Gore Canyon, where then it would turn north and follow the Yampa River towards Phippsburg, Steamboat Springs, and Craig, eventually passing through northern Utah into the Salt Lake basin.

The railroad never reached its namesake western terminus, though. Directly west of Denver lies the backbone of the Rockies, with no low passes. In an effort to use as much capital as possible extending the line westward towards possible sources of traffic, a temporary route was built over the 11,600 ft. Rollins Pass, with the intent of building the main range tunnel in a few years. This temporary route had numerous sharp curves and a sustained 4% grade, necessitating helper locomotives and crews on nearly every freight train. As if this wasn't enough to eat up any meager profits, winter on Rollins was a fiscal nightmare. The line would routinely be shut down for days or weeks on account of blizzards at the top, and frequently trains would become stuck in the huge drifts. Even plow trains would become stuck, only to have the plow trains sent to free them get stuck as well. Clearly the main range tunnel was needed, but there was no money left to build it.

Through many infusions of cash by both David Moffat himself and other investors, the line did eventually reached as far as Craig, CO, by 1913, but the DNW&P was already in bankruptcy, largely because of the expense of keeping Rollins Pass open. Even when it emerged as the Denver & Salt Lake Railroad, the Pass continued to plague the line with slow, unreliable, dangerous travel. A small road like the Denver & Salt Lake, with few sources of revenue, simply could not take the financial strain that operating Rollins Pass placed on the budget.

Thanks to a bizarre twist of government, the State of Colorado agreed in 1923 to build the 6.2 mile tunnel through the main range of the Rockies, at an elevation of about 9200 feet. In honor of the visionary who had first built this particular railway and done so much for Colorado railroading in general, the new bore would be called the Moffat Tunnel. It would be part of the Moffat Tunnel Improvement District and owned by the people of Colorado. In turn, it would be leased back to the railroad. Started in 1924, the Moffat Tunnel was completed four long years later, with the first train running under the mountain during February 1928. So successful was the tunnel that the line over Rollins was immediately closed, and no further traffic passed over it with one exception. During March of 1929, part of the Moffat Tunnel's roof collapsed, forcing the railroad to send a few detour trains over Rollins. After that, no further traffic went over the top. By 1935, the route was abandoned and was taken up shortly thereafter.

With the Moffat Tunnel being built, it was clearly necessary to connect the west end of the D&SL to something, lest the tunnel become a big, expensive hole to, essentially, nowhere... Since completing the original DNW&P / D&SL route to Salt Lake would have required another three hundred miles of new railway and would have duplicated the D&RGW's existing standard gauge route, it was decided to connect the D&SL from Bond, CO, 45 miles southwest along the Grand (now Colorado) River to a point on the D&RGW's standard gauge Tennessee Pass mainline near the confluence of the Grand and Eagle Rivers. This route, eventually connecting timetable locations known as Dotsero on the west and Orestod on the east, would be the Dotsero Cutoff. While it took some time to get it done, the route was finished in 1934 as a condition of the D&RGW's acquisition of the D&SL.

At that point, the modern Rio Grande standard gauge system that we all know was, for all purposes, complete. The D&RGW had formally taken control of the D&SL in the early 1930s, but it would remain separate, at least on paper, until 1947. At that point, it was formally folded into the D&RGW. The Moffat Route had become the preferred route west for Denver traffic, and west of Bond, the rest of the former DNW&P / D&SL route from where the Dotsero Cutoff split to the end of the line near Craig, CO, would become known as the Craig Branch - a key coal-producing line.

The Moffat Route would continue largely unchanged until the end of the Rio Grande on October 13, 1988. On this date, the Rio Grande purchased much larger Southern Pacific and then merged itself in as part of the "new" Southern Pacific. Only eight years later, on 11-Sep-1996, the SP itself would disappear into Union Pacific.

Today, UP continues to operate the Moffat Line, and BNSF runs trains over it via trackage rights acquired as a condition of the 1996 merger. Most through UP freight has been rerouted via Wyoming, but some does still flow across the Moffat. Mostly, UP uses the Moffat to bring unit coal trains to and from the North Fork? and Craig Branch. Passenger service still exists over the line, with Amtrak's California Zephyr providing daily service and the Ansco Ski Train running between Denver and Winter Park at select times of the winter and summer.