Interestingly, there is a railroad line that goes north from Vancouver, through Whistler and Pemberton, but takes a different route than Highway 99 to get to Lillooet[1]. The highway (aka The Duffey, as it passes Duffey Lake) gains and then loses something like 1000m of elevation, whereas the railroad line is much flatter and travels along Gates Lake, Anderson Lake, and Seton Lake. There are dirt/gravel roads there too, but I'm not sure if they are used in the winter.

The railroad route is not used much these days, partly because of all the trains that fell into the lake due to how windy (edit: that's windy as in lots of turns, not windy as in lots of air blowing over it) the track is (e.g. [2]).

1: https://www.openstreetmap.org/#map=10/50.5405/-122.6157

2: https://www.trailtimes.ca/news/cn-freight-train-derails-alon...