Here's an argument that keeps coming up: The light-rail tunnels that Sound Transit is digging under Capitol Hill are, basically, the same as the proposed deep-bore tunnel downtown—but you don't see tunnel critics complaining about the light rail.

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Sorry, no, they're not the same. Even if you lerv the deep-bore tunnel, they're different animals. Different engineering, different financing, different function.

"They are two very different projects," says Sound Transit spokesman Bruce Gray. "We are using similar technology, but the scale is very different between the two."

The twin bore tunnels under Capitol Hill (like the tunnels under Beacon Hill) are much smaller. At only 21 feet in diameter, they have an area of 346 square feet. The proposed deep-bore tunnel, according to the presumptive bid, would nearly three times the diameter. At 58 feet wide, that's 2,642 square feet in area—or a 7.6 times larger hole. The downtown tunnel is the widest deep-bore tunnel ever attempted.

More after the jump.

That's not to say smaller tunnels don't encounter problems. Some don't. Other do: Two of the machines digging the Brightwater sewage treatment tunnels in north King County (about 15 to 20 feet wide) got stuck in silty soil, driving up costs beyond their bid price; the Beacon Hill tunnels for light rail also ran over budget.

But if the deep-bore tunnel has these problems—and the odds are stacked that it will hit a snag—the ramifications are likely to be bigger, harder to solve, and more expensive.

Light rail is also financed differently. It's funded by a sales tax, and Gray notes that Sound Transit has the authority to extend the tax to pay for the project. Or, alternatively, the scope of the project can be reduced. In fact, due to a drop in sales tax revenues, Sound Transit has already cut back its plan. But light rail to the suburbs is a more scalable project because more of the components are expendable (remove a stop, delay an extension, build a less expensive station) without fundamentally compromising the project.

In contrast, the $4.2 billion deep-bore tunnel has a state financing cap on the project. And it's far less scalable because most components are vital to traffic management in the city's core. Left incomplete, if the money runs out, the tunnel project would leave unconnected streets downtown, unfinished portals, a dangerously unstable viaduct left standing, or even a tunnel that doesn't connect from end to end. Given that $300 million from the Port of Seattle isn't shored up, $400 million from tolling is considered unworkable (even thought it's needed to complete the project), and the contingency fund is more than half gone before the project has begun, the financing for this project is, at best, tenuous. At worst, it means there may not be enough dough to complete the basics.

And last: Seattle is long overdue for a rail system. There's no other way to build a station on Capitol Hill hill without digging a tunnel. Downtown, where we need to replace the capacity for six lanes from the Alaskan Way Viaduct, we have other options.