One of the minor disappointments as Central Link took shape was the lack of center platforms. Of 13 stations, only Stadium, Beacon Hill, Rainier Beach, and Seatac have these center platforms. The four DSTT stations share right-of-way with buses and so in effect must have outer platforms.
Center platforms are highly desirable at termini and at train-to-train transfer points, for obvious reasons. As the only currently planned transfer point, I’m hopeful that Chinatown/ID Station will get a center platform between now and 2020 to facilitate transfers from South to East and vice versa.
More generally, though, it seems prudent to construct these wherever possible because you never know where a terminus or transfer might occur someday. Rainier Beach is equipped to be a turnaround for trains because it has a switchover, so it’s fortunate it has a center platform. To speculate a little more wildly, in the distant future Link may more or less follow the RapidRide F Line from Burien to Southcenter. Tukwila Station would be an obvious place to switch between the lines. Unfortunately, for no apparent reason this was built as an outer platform station.
The other bonus of center platforms is that it makes it easy to double back if you overshoot your stop. Fortunately, Capitol Hill appears to be center-platform. Here’s hoping Sound Transit finds more opportunities to use this approach.