Despite its name, the SkyTrain in Metro Vancouver isn't entirely elevated; geography and the cityscape prohibit that. There are, in fact, three underground sections on the system where, if you didn't know better, you'd think Vancouver had a subway. One is the rehabilitated Dunsmuir Tunnel, which explains why the route is so weird downtown, and the other is a short segment under Clarkson Street in New Westminster, immediately around Columbia Station. I took this photo of a portion of that tunnel from the railfan seat on a westbound train. Here you can see a crossover connecting the two tracks; presumably, this is what was used during the service slowdowns a few months back, when the Millennium Line was in operation only between VCC-Clark and Columbia.
The third underground section, by the by, is pretty much the entire length of the Canada Line in the city of Vancouver.
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