tangent
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Pedestrian draws, especially that many in a short span, are probably going to be no-go on expense and maintenance complexity. That starts becoming a little bit Rube Goldberg-esque. It would honestly be cheaper to go super-tall over the dam at maximum mast height (as set by the Eastern Route commuter rail bridge) than to add moving parts. Super-tall has its own compromises because that's really, really fricking tall and will take complicated sets of switchback ramps to get up there. So don't start celebrating just yet. An unfavorable assessment could easily nix the straightest crow-flies route across the dam.
Sidewalk on the Eastern Route bridge is a lot more intriguing since the bridge has already got concrete emergency evacuation shoulders on it and tracks spread 2 feet wider from each other than on land. It's possible that modification of the side walls can cobble together enough space for a side path on 1 side + security fencing, and that the tracks can be compacted together a couple feet. Obviously if there's the slightest concern about safety margins that high up over the water it's an instant-reject from the T and FRA. For one, the superelevation on the Somerville side may be the reason why the track geometry has them spread 2 feet further apart. But it's worth a full-on investigation to answer that feasibility question because this is potentially the cheapest option of all if those shoulders are safely modifiable.
The important thing in all this is that they're going to try for it and at least answer that feasibility question definitively...and do so with proactive public-private partnership. That's a big deal by its lonesome that can only lead to more good things.
All of the options are going to cost some multiple of millions. The 290 foot long North Point pedestrian and bike bridge seems to have cost about $10 million out of a $25 million dollar project for very rough comparison purposes.
Yes, usually you avoid draw bridges because of the added expense. But in this case you could end up with a better pedestrian and bike experience with a level crossing compared with a climb up to an elevated crossing, so at least worth considering.
The standalone option is also interesting. I would want to see about the feasibility of building tall over or alongside the dam potentially as a way of reducing the length of the necessary span and aligning with the existing access road there. Looks like you would have to get about 30 feet tall above the damn to match the 45 foot clearance of the train bridge.
Also, I hope they make some mention of connecting the last link of the river walk (on both sides of the river) as discussed earlier in the threads. Compared with a new pedestrian crossing closer to Assembly, that is a long long walk down to the Alford St Bridge and across and back over, but completing the river walk is something that should be done either way to really pull this area together.