Good question. Here’s my stab based on “totally reliable” sources:

Wikipedia says it’s 95 ft per car, so you need 380 ft for a 4 car train, which gives you 800 people per train and as many 24,000 per hour, at absolute maximum capacity.

Dead reckoning on Google Maps tells me that most downtown blocks are 150-200 ft. So it’d be 2 cars at most, meaning you could run 12,000 people per hour at 2 minute headways.

If you want the Ballard end of the line to go anywhere beyond Ballard, that’s probably not enough capacity, but I’ll defer to someone who knows more about the relevant population densities and commute patterns.

Anyway, at some level, I think the deeper question is whether a Ballard-West Seattle line needs to be engineered to the same standard as Central Link. Personally my standard is whatever gets something that can average 35 mph built soonest.

- Tunneling under fourth (or whatever) isn’t demonstrably cheaper than under second; the key to my idea is that you get a large part of the costs for free, not because I have a particular love for using the waterfront.

- I support the idea of using some sort of moving walkway/escalator to get to 3rd Avenue. Can it really cost that much?

- If you take a street (1st, 2nd, and Western were all mentioned) and hand it over to trains, you (1) have huge political problems in taking away precious car capacity; and (2) limit your trains to the length of a city block, like Portland. No thanks.

- Don’t like the streetcar idea? Then send West Seattle buses through it. I’m flexible, as long as we can run LINK down the road.

- “Integration with light rail” is not a significant reason that the monorail died.

This gives a very nice gradient and alignment, zero tunneling costs, and a transit spine easy to reach from every important part of downtown Seattle.

What is happening in Seattle is that the automobile routes are being de facto dead-ended short of the city core. In the 50s Seattle was built with automobile routes running right through downtown. Merchants of the time thought this was how they would keep their sales up.

But Seattle is just too narrow to handle the automobile traffic of a region that has added a million residents. When I moved in to Seattle, going through downtown was often the fastest way to get somewhere. Now it’s usually the slowest.

So hand-wringing about keeping the cars moving is not needed- that ship has sailed. As someone said on another thread, the best solution for the traffic snarls right now is just to close a few freeway ramps.

And the best solution for the next north-south route is to take a right-of-way that already exists and use it for a form of transit that actually works.

Here’s an idea. Let’s use Martin’s waterfront tunnel, but add a pedestrian tunnel from the waterfront to Link. It’s around a 1000 foot walk – about 3 minutes (less if we add moving sidewalks). We’d still need to build a bored tunnel, but it would be much shorter and smaller.

Add Underground connections to each station, crossovers, and move Central Link the the 4th Ave Tunnel. The tunnel on 3rd would be for W Sea-Ballard, and buses.

Click here for google map

Again, this is a potential, all surface, super-cheap route from Burien-Greenwood.

http://maps.google.com/maps/ms?ie=UTF8&msa=0&msid=118219652458657252175.00044b1c0d9e9a9e2e054&z=11

Here’s a map I made for Future Rail in Seattle

I wouldn’t be opposed to trying to find some kind of surface light-rail solution for West Seattle-Ballard as long as it-

-Connects to Central Link at some point (MINIMAL walking, otherwise people WILL NOT make the transfer)
-Is at least as fast as a bus would be.
-Serves the same basic area that the Green Line would have.
-Takes maximum advantage of the infrastructure (if we need proper light rail vehicles over streetcar vehicles than so be it)

I think a surface solution like they have in Portland could be the solution we need for this particular corridor.

Central link is different. It goes far enough that it needs large sections where it can pick up speed.

I’m not so sure about West Seattle, it may need to be elevated if it is to extend to Burien, but if we are just going to Ballard or even Greenwood, I think a surface corridor could probably be established.

Now streetcar projects can evolve into Light Rail, and I would start with the Streetcar, but the cost of the tunnel in Downtown, although needed, I am just not sure. I am one that thinks the existing DSTT should have been wider, or an extra set of tunnels in place for future conversion, it probably would have been cheaper then.

Also, in Allied Arts’ presentation on a vision for the Viaduct-less Waterfront, they had an interesting CGI, showing a streetcar on Madison Street. Wonder if that would be possible.