There is a technical term called "
SWAG". This post will be highly SWAGful.
The
2019 NSRL Feasibility Reassessment made the bizarre choice (one among its many bizarre choices) to do cost estimates that pretty wildly failed to be apples-to-apples
when comparing the two-track vs four-track Central Artery alternatives:
In addition to doubling the number of tracks and adding a resultant second portal, they also added a third station (and did so literally as a parenthetical). As far as I can tell, no justification is given for adding Central Station to the four-track but not the two-track alternative.
Unsurprisingly, the four-track total costs they present are wildly higher than all other alternatives:
Of note: the four-track alt is the
only alignment they evaluated which includes a South Bay portal to provide NSRL access to Fairmount service. While I could probably be convinced of the usefulness of a two-track NSRL rather than four-track (obviously preferring four-track), imagining an NSRL
without a Fairmount connection seems like a total non-starter to me, for a host of reasons.
What I'd actually like to see are estimates of:
- 2-track 2-station 2-southern portals alt ("2-2-2")
- 4-track 2-station 2-southern portals alt ("4-2-2")
So let's do some SWAGging...
Portals: this one is weird. All four alternatives are estimated at roughly $680M ± $8M. Despite being a 33% increase, adding a fourth portal at South Bay only seems to increase the cost by 1%.
In fact, looking through the document, as far as I can tell
there is no reference to a South Bay portal at all in the costing estimates.
Setting that aside, for the sake of conversation, let's assume that the $680M figure is valid for three portals (Back Bay, Fitchburg Line, Lowell/NR Line). A 33% increase to four portals brings us to $904M, or
$226M per portal.
Stations: the two-track CA alt estimates $445M for what is essentially two stations (North Station + South Station in 1 tunnel) --
$222.5M per station. The four-track alt estimates $1345M, which seems high but is in fact almost exactly the same cost per station, since this alt is essentially
six stations (North + Central + South, each with stations in 2 tunnels) --
$224M per station.
Tracks + Tunneling: these figures seems broadly reasonable. The two-track alt estimates $1003M, and the four-track estimates a bit more than twice that at $2384M (probably attributable to the increased complexity of the flying junctions at the northern end). Per mile, that's $371M per tunnel-mile for the two-track and $433M for the four-track -- let's call it
$400M per tunnel-mile. The trackwork looks like it is about
$38M per track-mile.
Risk Allowances: for all four tunnel alternatives, multiplying the estimated Direct Costs by
362% seems to land you remarkably close to the Assessment's estimated Final Costs.
Adjusting the costs: (some of the addition is slightly off due to rounding, but I think it should "only" be $5M, give or take)
Adjusting the
two-track estimate to include a second southern portal and an extra .63 miles of tunnel to South Bay:
- Tunneling: $1002M --> +$252M = $1254M
- Stations: $446M
- Trackwork: $104M --> +$24M = $128M
- Portals: $680M --> +$226M = $906M
- Allowances: $146M
- Layover Facilities: $34M
Original direct cost total: $2412M
Adjusted direct cost total: $2914M
Total estimated cost for "2-2-2" alt: $10.5B
Adjusting the
four-track estimate to exclude Central Station (and apparently add a second southern portal):
- Tunneling: $2384M
- Stations: $1346M --> -$448M = $898M
- Trackwork: $207M
- Portals: $680M --> +$226M = $906M
- Allowances: $151M
- Layover Facilities: $34M
Original direct cost total: $4809M
Adjusted direct cost total: $4580M
Total estimated cost for "4-2-2" alt: $16.6B
(If it turns out I'm wrong about the costs for the South Bay portal being missing, you can knock $818M off of the "4-2-2" alt, for $15.8B.)
~~~
Knowing nothing else about the project, using a general estimate of $1B per route mile (broadly consistent with data from
other projects, see also
their appendix) in direct costs, the 2-2-2 would be estimated 2.7+.6 = 3.3 route-miles --> ~$3.3B; the 4-2-2 alt would be 2.7+2.8 = 5.5 route-miles --> $5.5B. Those estimates are generally in line with the most detail estimates above (prior to applying the risk/contingency multiplier).
So, to be clear, these estimates are indeed SWAGs -- ultimately wild ass guesses, but with at least some data and logic in support.
