Interesting- wonder if these were used in the construction trams for the Moffat Tunnel rather than for the actual railroad. They would have been probably too light for the mainline. However they did replace the rail in the Moffat Tunnel in the late 40's or 50's I remember seeing a photo of the Fairbanks Morse H15-44's doing it on a rail train (I think it was in the Rio Grande Diesel Locos Vol 1- but my copy was destroyed in storage) as they were the only ones that could handle it. That would have placed it before the arrival of the GP7 and SD7's. Points to ponder.
Kurt, a correction. Moffat and the Denver Northwestern & Pacific / Denver & Salt Lake did not build the Moffat Tunnel; the Moffat Tunnel Improvement District did.
Since the UP-CP transcon was constructed to the north over Sherman Hill in 1867-68, Denver was ever frustrated not being a stop on a direct transcontinental route. We know that a group made an attempt to construct a tunnel under the Continental Divide west of Denver (and south of Loveland Pass), but did not get very far. David Moffat, ever the Denver booster put his money down and started construction of the DNW&P early in the 20th C with the aim of building a line to the Pacific (or at least Salt Lake City, er Craig). As we know, the line was surveyed over Rollins Pass which proved to be an expensive segment of railroad to operate.
Denver folks sorely wanted a tunnel under the Divide, but no one had the cash to fund construction. Turning to government to solve the problem, the Denver folks were thwarted by southern Colorado interests. Pueblo was very happy with its position on the D&RG-MP transcon and had no desire to have this position compromised by Denver. Then the Arkansas flooded in 1921 causing vast damage to Pueblo and the rail yards. Now it was Pueblo's turn to appeal to northern Colorado for help. And help was provided...on the condition that southern Colorado would support construction of a tunnel under the Divide west of Denver. You will note the 'start date' on the East Portal of the Moffat Tunnel is 1923? Enough said: the Moffat Tunnel Improvement District was created.
There is a saying here in Colorado that, "water flows uphill towards money." The vehicle to fund and construct the Moffat Tunnel was actually a water diversion tunnel which is the pilot bore. To this day, water is diverted from the Upper Colorado basin to the north metro area. In fact the capacity of Gross Reservoir is being increased to accommodate growth in the metro area.
For years voters in a handful of Colorado counties got to vote for Moffat Tunnel Commissioners, and I recall that one of the mandates of the District was that railroads using the tunnel were required to provide passenger service to the west slope. This may have influenced the D&RGW's decision to continue passenger service independent of Amtrak for many years. Also, any railroad could use the Moffat Tunnel--they just had to get to one or the other portals. The construction bonds were paid off in the early 90s and the District disbanded in the late 90s. Sometime between the Espee bought the railroad tunnel and the water tunnel was sold to the Denver Water Board.
I suspect rails from the South Park were used to construct the tunnel, rather than as for the actual standard gauge railroad rail. But who knows? All of the above is greatly abbreviated, and if you want to know more, start Googling!