A single downtown tunnel is completely possible and provides the best outcomes
In his recent article, Martin Pagel outlined why a single downtown tunnel is a win-win. I'll emphasize mainly that transfers are crucial in a world where the suburb-to-downtown commute is no longer as common as it was. And transfers are a million times better when one only has to switch platforms in a tunnel than having to get out of a station, walk some distance, and get into another station.
But most importantly, using a single tunnel for the planned lines in ST3 is not a transit-nerd fantasy but completely feasible. Previous discussions on why a second downtown tunnel is needed have focused on capacity. According to Martin H. Duke's 2015 interview with Marie Olson, Sound Transit's Link Transportation Manager for Operations, the capacity of the existing tunnel should allow for 40 four-car trains per hour, or a headway of 90 seconds. Split between the three lines this results in 4.5 minute headways per line - significantly more capacity than needed for ST3 (planned 8-minute peak headways per line)! This particular interview points out that ventilation is not an issue either. Some work may still be required: ST3 project C-07: Transit Tunnel improvements enabling increases in system frequency estimates that approximately $20m will be needed to enable higher than 3-minute headways in the tunnel. That is negligible compared to the billions needed for a second tunnel.
It's important to realize that this path - upgrades rather than completely new construction - is the path San Francisco took with the 2009 MUNI signaling upgrade. That's an important regional precedent that cannot be dismissed as happening in vastly different conditions. MUNI achieved a 50% effective capacity increase for $104m, easily 20 times less than the cost of a new 3.5-mile tunnel. They went from 30 trains per hour to 45 trains per hour with a design capacity of 60 trains per hour. Yes, that is a system supporting 1-minute headways (!) for light rail trains. Not to mention that MUNI trains share the road with regular traffic for much of their route (like the First Hill and SLU streetcars) and arrive at the tunnel at irregular intervals, making their problem vastly more difficult than ours.
Sound Transit needs to make it their #1 alternative to leverage the existing downtown tunnel. Not just for better transfers and avoiding disruption in the CID, but because the time savings of constructing a very complicated downtown megaproject will allow for new lines and extensions to open sooner, and will help pay for any cost overruns. And if we are lucky, there may be funds available to enhance other projects.