This was not taken in Alaska but since it was spotted at CN’s barge slip in Prince Rupert I think it safe to presume it was headed to Alaska

My guess is that it has something to do with logging. A few loggers here on Vancouver Island converted Sherman tank chassis  into portable spar-trees. There is one on display at at the BC Forest Discovery Centre in Duncan, just south of here. As for what I presume were pontoons (stored on top) I can only guess that this thing was meant to be amphibious!

Additional information provided by John Stutz - "This appears to be the hull of a WWII Marine Corps amphibious tracked landing vehicle, modified by addition of a pair of side mounted pontoons.   The pontoons have been dismounted and tied down atop the vehicle.  This may have been done in order to fit it into AML's shipment loading gauge, to avoid shipping surcharges on excess width, or to reduce a volume based shipping rate computed from the maximum length, width and height. Regarding the latter possibility: White Pass during the 40's & 50's cut out the domes on tank car shells purchased from UTLX, shipped the dome in its tank shell, and welded it back on after arrival.  This reduced their shipping volume by about 1/5. In this case, the shipping volume has been reduced by about 1/4."