Looking at the photos of SVRY 19 and 20, they might have been assisted from a (rail mounted?) crane on the dock. It is interesting that they were loaded crosswise as deck freight - not lengthwise or down in the hold. Though that crane to the right in Seattle looks far too light to have lifted that much weight. Or maybe a combination of a dock and ship's cranes?
Note the engine at Skagway is still on that wooden sled. Did they actually skid the engines sideways off the ship at Skagway? They tended to do a lot of that type of heavy or oversized moves in those days without a second thought. Just wait for the tide to have the ship just a little higher than the dock and slide it off.
That may be also how they loaded them at Seattle.
As for the 70 class, they may have just thought it easier to move them as a kit especially considering the distance. Did they rail to the west coast and then by ship to Skagway, or did they go onto a ship off Baldwin's dock and go through the Panama canal all the way to Skagway?