The Denver, Northwestern & Pacific built west out of Denver under the direction of David Moffat in 1902. Their goal was to provide a route directly west to Salt Lake, as opposed to the D&RG's roundabout standard gauge line via Pueblo, Salida, and Tennessee Pass that added some 200 unnecessary miles to the trip. Part of the chosen route, as surveyed by H.A. Sumner, was a six mile tunnel under the backbone of the Rockies. Given the era, there was no practical way to build such a bore, especially with the limited funds afforded the DNW&P.
Moffat considered a shorter, 2.6 mile tunnel located at a bit over 10,000 feet. However, since his railroad had no source of revenue carloads yet, he felt funds would be better poured into stretching the rails westward towards the resort at Hot Sulphur Springs and the coal mines near Phippsburg. Consequently, a cheaply constructed line was erected across the Rockies, crossing the Divide at Corona, atop the 11,600 foot Rollins Pass. The line was cheaply built, with sharp curves and a steep (4%) continuous grade, but it was only intended as a temporary solution for a couple years until the money was available to build the tunnel.
A few years dragged on into a few decades. Each winter, weather over Rollins would at least tie up men, money, and machinery trying to battle the snow and the grades. Inevitably, though, they would lose, and blizzards would shut the line down for days or even weeks at a time, sometimes stranding trains on the pass. This short stretch of the line would bring the whole DNW&P, or later the D&SL, to its knees, eating up any meager profits the railroad might manage to make, and was largely the reason that the line was perpetually on the verge of receivership. It was this constant drain, along with the desire to push the road west in search of more traffic that caused David Moffat, having poured every last dime into the route, to die nearly penniless in 1911.
Finally, in the early 1920s, a break came. Thanks to some political deals between the Denver and Pueblo representatives in the Colorado government, the state agreed to build the main range tunnel as part of the Moffat Tunnel Improvement District. It would then be leased back to the railroad. Construction started on the optimal, 6.2 mile tunnel in 1924, and by 14-Feb-1928, the route through the mountain was complete.
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.
Source: USRA D&SL Timetable 19, from 6-Jul-1919, as reprinted on page 67 of Denver and Salt Lake Railroad: 1913-1926 by P.R. Griswold
Rollins Pass is still accessible today via a very rough road - not really requiring four-wheel drive, but definitely requiring high clearance. The snow usually hangs around until July and starts again in September, giving any explorer only a few months of reliable access. Additionally, summer thunderstorms make the exposed climb over the summit dangerous even during the few good months on account of lighting and hail. I've ventured up the east side, and intend to do the west side next summer. You can see pictures of what remains today in my trip report below: