The Main Range Tunnel, now officially named the Moffat Tunnel by the legislation in honor of David Moffat's vision, went through a series of court challenges until the US Supreme Court ruled the legislation constitutional on 11-Jun-1923. The Denver & Salt Lake wouldn't be building or even own the tunnel; that would be purely under the jurisdiction of the Moffat Tunnel Commission. However, not wasting a single moment, the D&SL began to haul supplies of all manner to both tunnel faces - near Newcomb on the east and Irving on the west. The chosen tunnel was 6.02 miles long, at an elevation of 9,100 feet - the most ideal of all the previously surveyed Main Range tunnel prospects. (See the old map at right for all three proposed routes.)
Tunnel construction was far from a walk in the park, however. Geologists of the time believed that the tunnel's path would primarily take it through very solid rock, with only a small amount of glacial drift covering the ends. What they actually found inside the mountain was a horrid mess of shattered rock and flowing water, crossing three different fault lines over its length. The west end was the worst, where the instability plagued digging from day one. Contrary to expectations, nearly the entire thing needed timber lining, severely limiting the rate of progress. The east end, by contrast, was the relatively solid rock expected, and needed only minor timbering or sprayed-on concrete. This was only a false hope, because on 14-Feb-1925, just past the apex of the tunnel, the eastern pioneer tunnel struck a fissure in the rock. Unlike the ones before, this one was direclty connected to a glacial lake sitting 1500 feet above the tunnel. Flooding ensued, setting back operations by weeks. This was to be the course of things for the eastern bore throughout the project, with a similar event happening in the main railroad tunnel a bit over a year later, on 28-Feb-1926. Again in April (due to a power failure and stopped pumps), water backed up and flooded the eastern bore from the apex to the western face.
And so went the tunnelling, complete with unstable rock and incessant flooding, until on 18-Feb-1927 at 2010h, the tunnel was finally holed through after more than three years of digging. The two bores met to within 1.5 feet horizontally and 3.5 feet vertically - not bad for a 6 mile tunnel without the benefits of lasers or modern surveying equipment. Almost exactly a year went by, finishing off the tunnel, timbering those sections that needed it, laying track, and preparing the ventilation system. Some argument (especially from the Tunnel Commission engineers) was given to electrification, but in the end economics and steam won out - forced ventilation was far cheaper both initially and operationally than a short chunk of electrified railroad. So forced ventilation it was, and on 14-Feb-1928, the first unofficial steam powered train, lead by D&SL 120, made its way through the bore. The official opening wouldn't occur until 12 days later, when amidst much fanfare, D&SL Mallet 205 made its way westward into the tunnel at 1320h, with passengers in tow.
The tunnel was truly a marvel - 6.21 miles bored directly under the Continental Divide, cutting off 2500 ft of climb and almost 23 excess miles of track. At a cost of 15.5 million dollars and 19 lives, it was the most expensive (per foot) large tunnel in existance at the time. In addition to the benefits of the main railway bore, the parallel pilot tunnel (the one drilled to facilitate access to multiple tunnel faces) was converted into a aquifer for hauling Western Slope water over to supply the city of Denver - a purpose probably equally important with rail traffic today. At this point, all of the major pieces of the modern Moffat road were in place, save one. The Denver & Salt Lake was still a railroad to nowhere. While the coal from Routt County, the oil from Craig, and the regular passenger and livestock movements certainly provided revenue, Moffat's dream of a Denver-Salt Lake linkage was yet unrealized.
Aside from a few days of reroutes in 1929 when the Moffat Tunnel's roof collapsed, the old Rollins Pass line never saw service again. On 14-May-1935, the Denver and Salt Lake received permission to tear up the route, and by 12-Aug-1937, the job was essentially complete. It was handled by trucks, as the right of way had deteriorated too far (roadbed damage, collapsed tunnels, etc.) for it to be done by train.
Notes & Trivia
The Moffat Tunnel had car washers installed inside both portals at one time to wash passenger equipment as it exited the tunnel so as to clean off any grime accumulated in the tunnel.