I would be surprised in the USG locomotives would not go through the tunnels if a Alco 101 would. These locomotives were built for the White Pass which closed before they could be delivered. Thus, the loco's had to clear the same clearances the other White Pass loco's did. The White Pass has two tunnels like the C&TS.
The fourth of the batch, the 114, eventually made it to Skagway where it is in work service and seems to do just fine. 111, 112, and 113, went to USG. 113 was destroyed in an unfortunate "derailment" as the D&RGW would say.
C&TS would be better off with one of the supercabs as the Alco's were supposedly despised by the White Pass train crews and track crews. Because of how their trucks were mounted they were very difficult to turn and really tore up the track on the hill from Skagway. They supposedly did fine on the relatively flat and straight track from Bennett to Whitehorse.
The supercab MLW DL535E have Alco 251D prime movers with GGTA19PB1 generators. They weigh in at 216 Metric tons and are 53'5.5" long. They develop 43,200 lbs. of tractive effort, 64,200 lbs. of starting tractive effort, and develop 1200 horsepower. These loco's carry 1000 gallons of fuel. They also have different trucks and are much easier to turn and do not tear up the track like the Alco's.
I would say that one of these locomotives could handle the work train and rescue duties for the C&TS. However, if USG does phase out their operations, I would be surprised if the White Pass did not go after these locomotives. They purchased back all of the Alco's they sold to a South American railroad.
Personally, I think a 90 class would be better on the track but I seriously doubt the White Pass would let go of one of these.
