What is this?
Two substantial SkyTrain extensions are currently underway in Greater Vancouver, further expanding Canada's longest metro system. The Langley extension will introduce rail service to the eponymous outer suburb, adding eight stations to the busy Surrey branch of the Expo Line. The Broadway extension, an urban subway to Arbutus Street, will form the first phase of the Millennium Line's extension to the University of British Columbia, replacing Canada's busiest bus route. This page contains some back-of-napkin sketches of these extensions' possible broader implications on SkyTrain service.
The SkyTrain system's current service pattern includes two pinch points that limit capacity on the Surrey branch and the Millennium Line, hindering the extensions from operating at their full potential. Not all Expo Line trains can serve Surrey, as this line splits in two at Columbia station in New Westminster. Those trains that do not enter Surrey turn north to serve Lougheed Town Centre and Simon Fraser University, sharing tracks with the Millennium Line along the way. This branch consumes some of the Millennium Line's capacity, not leaving enough for the rest of that line to enjoy the system's characteristic frequent service.

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Production problem: no-build solutions
The simplest way to unlock full service on the Millennium Line is to truncate the Expo Line to Lougheed Town Centre. This solution takes advantage of Lougheed station's third track to terminate Expo trains without physically interfering with Millennium Line operations. Millennium trains would be free to operate as close to each other as the system allows, from Vancouver to Coquitlam.
This approach has some negative effects on multi-seat journeys. Many trips to Simon Fraser University become longer, as Lougheed station is farther than Production Way station from campus, and Expo Line riders must spend more time on an extended connector bus. The Burnaby Mountain Gondola will serve Production Way, incurring a new transfer for Expo Line - SFU journeys. Additionally, changing from the Expo Line to the westbound Millennium Line at Lougheed involves a down-and-over platform change, compared to today's same-platform connection at Production Way.
These effects are eliminated by the reintroduction of "pretzel" service. Before the Evergreen extension opened in 2016, the Millennium Line travelled between Waterfront and VCC—Clark, crossing over itself at Commercial—Broadway. Such a service preserves one-seat rides between Columbia and Production Way, extending these to Brentwood Town Centre and beyond. By sending every train through Production Way to Arbutus, the entire corridor can make use of its full capacity.
This solution comes with its own set of complications. The pretzel service further ties Expo and Millennium operations to each other, introducing risk of delays cascading throughout the system. The pretzel shape itself is difficult to communicate, as a train's bearing and effective destination change along its route. Splitting it into an independent line identity more faithfully represents the service at the expense of system legibility, especially at Commercial—Broadway and points west. Rolling it into the Expo and Millennium main lines simplifies system topology, but may prove unintuitive to riders in the east.
Columbia conundrum: full relief
Note that the above solutions only address the Broadway half of the problem at hand. The Surrey branch of the Expo Line is the busiest of SkyTrain's outer segments, and ridership will only grow with the Langley extension. Moreover, Columbia station itself is unfit to serve as an interchange, as passengers must travel between side platforms via cramped mezzanines.
While expensive, an expansion of Columbia station removes both of these constraints, allowing every Expo train to serve Surrey and adding interchange capacity at Columbia. A single-track terminal with Spanish-solution side platforms sits elevated above the existing station trench, with direct vertical access between each stacked pair of platforms. A branch of the Millennium Line travels between Arbutus and the new track at Columbia, accessing this via a short viaduct along Clarkson Street.

Columbia station platforms | CC BY 2.0 Dennis S. Hurd

Diagram of Columbia station expansion as proposed above

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