The Canada Line, from what I can see, offers no room for expansion. The reason for this is the shortsightedness in the size of the stations along the line. The stations can only accomodate the two-compartment trains currently in use. There's no room for longer trains.
The Expo Line, on the other hand, built way back in 1985 for Expo 86, has stations that allow for up to five compartment trains. That means over 25 yrs. ago they had the foresight to design stations that would be able to accomodate longer trains as the need arose.
When the Expo Line was launched there used to be just two-compartment trains running on it. During rush hours they'd add two more. Over the years, as ridership has increased, four-compartment trains run all day. These, by the way, are the shorter compartments.
The new trains, the same as the ones in use on the Canada Line, have longer compartments. Yet the Expo Line can still accomodate four of these longer compartments, and there's still room to add a fifth compartment when necessary.
The Canada Line stations can only take two compartments, and that's it. The Line is already carrying the ridership they'd projected to reach around 2021. And it's only been running for three years. How will they accomodate further increase in ridership?
They can't add more compartments due to the size of the stations. And they can't add more trains, from what I can gather. The trains are already running at 3-minute intervals, so how many more trains can they add?
Here's another advantage the Expo Line has that Canada Line doesn't. Because the Expo Line had the capacity for longer trains they were able to stretch the line further into New Westminster and on into Surrey. They were able to add the Millenium Line to connect with the Expo Line. As they reach further into Coquitlam and Port Moody they'll be able to feed into the Expo and Millenium Lines.
All these doors are shut to the Canada Line. If in the future they want to take the Line further into Delta or White Rock, they'll have to build new lines because Canada Line will already be over capacity with no possibility of increasing train sizes. Ditto if they build a line along Broadway. Connecting a Broadway Line with Canada Line will bring it to overload.
Unless I'm mistaken in my observations, I see this as a marked shortsightedness in the planning and design of the Canada Line. In fact, I see a return of the old 98 B-Line buses to help relieve the overload on the Canada Line.