The decision to pursue BRT is being said to be due to cost: the claim is that light rail would cost some 3x more than the BRT (which is being priced at $75 million). That this is debatable is already well-known here, especially since the BRT proposal being floated in this plan involves buying special equipment, one of the primary cost drivers of the light rail proposal.
Remember, the engineering and the subroadbed are already sunk costs in the City Branch. This means that the primary costs for light rail are (a) rails, (b) trains, and (c) electrification (if desired). (See update below.)
That said, BRT has two clear advantages, at least in the near term, over any rail proposal.
The first is obvious. Both the light rail and the light metro proposal were greatly handicapped by length. The "natural" light rail alignment is from 40th and Girard to 11th and Bainbridge via Girard, the City Branch, and 11th/12th; the light metro ran in the branch from Girard to 13th, where it segued into a new cutoff tunnel linking it with the existing Ridge Spur at Noble. Neither would have been longer than three or four miles--nowhere near enough to develop a full line's worth of ridership.
The second is a little less intuitive, particularly with the masking effects the specialized equipment provides: this is that the City Branch can act as a funnel for routes to the north and northwest, much as Woodland Ave. and the trolley tunnel that subsequently replaced it act as a funnel for routes to the south and southwest; the Branch's outlet is also such that routes can subsequently fan out to cover a broader swath of Center City destinations than any rail proposal.
To that end, I propose a 2.5 route system maximized for frequency, and thus ridership.
The first route is the 32, routed into the City Branch instead of the circuitous alignment it has through Fairmount, improving travel time by 5 or 10 minutes. It would run on a 15 (preferably) or 20 minute headway throughout the day, and degrade to 30 minutes after 9:00 or so.
The second route, which I am calling the x40, is a heavy modification of the current Cultural Corridor BRT proposal: Where the current proposal is merely planned to pass by cultural destinations, to the detriment of ridership, I extend it to St. Joe's to tap the Main Line's ridership potential, and twist it down to a Chestnut/Walnut alignment to Penn's Landing to hit eastern Center City's main employment centers: Penn and Jeff Hospitals and the offices around Independence Mall. (Penn's Landing, by the way, is little more than a convenient bus terminus.) Like the 32, this route would run 15- or 20-minute frequencies.
This ensures that a bus passes along the Branch every 7.5-10 minutes, frequent enough to say one will be along shortly.
Finally, the half-route would be the 27, which would be routed off the Schuylkill at Girard and onto this route at peak periods, when the Vine is a zoo. It would run express through this alignment, the shaved minutes more than enough to pay off waiting at the lights as it traverses Girard between the Branch's terminus and the Schuylkill Expressway.
A last element: at my table somebody suggested moving the Phlash to the Branch, which was echoed and supported by the planner at the table--indeed, it appears to be a major reason the City is pursuing the project. The way I understand it, the Cultural Corridor proposal (my emended x40) is proposed to supplant the Phlash, not supplement it; part of the idea is that the City Branch offers a convenient replacement to Parkway bus routes. This presents a problem w/r/t the 38, whose Mantua coverage element would be eliminated by a Branch move, but not for the other routes. It may be better to pursue a surface Pennsylvania Ave. alignment instead for this route instead, as its main ridership base resides in Mantua.
UPDATE: Since several people seem confused by my line of reasoning involving equipment procurement, let me explain it this way. (1) The City's current City Branch proposal involves procuring equipment specific to it. (2) Costs for buses and light railcars in this situation are relatively equivalent (buses cost less, but for the type being proposed, not by an order of magnitude). (3) The City claims the BRT proposal is 1/3 the cost of light rail. (4) (Assuming the light rail would be electrified) this implies the primary cost difference between the two is electrification. (5) The City's BRT price quote is $75 million. (6) That implies the cost of electrification, according to the City, is $150 million. (7) I find that doubtful; the needed electrification would be able to piggyback off of the (active) 15 and (dormant) 23 networks, largely, if not completely, already in place.
So the problem I'm having is that the City is trying to have its cake and eat it too. For the City to believably claim that BRT is 1/3 less than light rail, the BRT cost would have to be limited to just the busway. In the current proposal, it's not.
I think I attempted the original (and yes, tortured) line of reasoning to approach this issue another way--by finding a way to equivocate equipment purchases--in that attempt, by tying equipment purchases for this project to SEPTA's impending trolleycar replacement.
By the way, if it's not already obvious, my major reasons for support of the BRT proposal here are because of the brevity of the ROW being discussed, and because BRT does offer superior alignment and routing options per x unit of capital (not operating) cost.