Boston has been considering linking the two halves of its weird bus/subway hybrid Silver Line for a while now. The so-called Phase III project has gone through several designs and redesigns, one of which called for a 0.6-mile long tunnel for a cost of $780 million ($1.3B per mile). More recent proposals were saner and cheaper, but people who live downtown complain about the added traffic and noise of articulated buses that hardly anyone uses anyway.

But that got me thinking: That’s just a bus tunnel. How does that cost (nearly a quarter-million dollars per foot!) compare to historical Boston subway projects? The red line was extended in the early 80’s from Harvard Square to Alewife. That 3.2 mile extension cost $579 million, including three new stations. In today’s dollars, that’d be about $459 million per mile, or about one third the price of that Silver Line plan. Sure, a tunnel through downtown would be a more complex project, but that seems extreme.

And for the anti-drivers in the audience, for the cost of the Big Dig ($14.6 billion, and it’s not yet done), Boston could have built 31.8 miles of subway including 30 new stations. That’d reach to Nashua.