Kenmore Loop only goes C/D to C/D between the outermost tracks of the station. The B takes the two center tracks. Loop pulls out too close to the platform for there to be new crossovers accessing the loop from the inner B tracks, and the cross-cutting traffic would induce delays.A UR question....
So assuming use of the GJ and a Harvard spur, burying the B and a D/E connection at BV, what are the technical issues about connecting the UR by branching off the B just west of Kenmore and connecting with the C/D tunnels underneath and use the loop? I have seen a lot of talk of alternatives that all seem to have fatal flaws, but I have seen little about using the loop. I know that it is not the straightest route in the world, but still faster than the 66, especially with the fresh hell that Harvard will create in Allston.
Structurally, it would be heinously disruptive to try to blow up the loop and reconfigure it for a B-to-D/D-to-B direction, or spread stuff out to try to create a new 'lower' loop underpinning the tunnel just east of the station. Also, a B-to-D loop would be much tighter and slower than the current one, while probably needing to overcompensate for the changed geometry by bulbing-out more under the south side of Kenmore Square (at increased building mitigation risk) rather than staying centered under the Square like the current loop.
Now, UR thru-running loopage might've be a desireable thing if the south half of the Ring were also light rail branching off Longwood or Brookline Village. But the lack of available ROW's south vs. near-perfect string of ROW's north means you're probably just dividing the thing in half as northern LRT and southern BRT out of necessity and changing upstairs/downstairs at Kenmore to get between the NW and SW Ring quadrants. It's the only way to ideally deploy modes, and since a radial line is intrinsically a quick-on/quick-off transfer centric affair there's extremely few people who would be riding it for more than a quadrant at a time...much less riding end-to-end like it's Alewife-Braintree warped into an oval.
For that reason, a hop-across at Kenmore to a Longwood-fetching D satisfies the 66 audience plenty well. In a case like that, existing Kenmore Loop serves up an opportunity to strengthen the transfers. We have our E-to-D surface connection, and its usefulness is primarily going to be peak-period augmentation of Huntington Ave. service like the old days pre-1985 when Heath-Lechmere turns interlined simultaneous with Arborway-Park turns. Only with the connection at Brookline Village you can--instead of short-turning or running west to Reservoir--opt to turn east off of a side platform on Pearl St. onto the inbound D, and loop at Kenmore. Then reverse back to Brookline Village, back onto the connector, and back down Huntington as a regular E. It adds zero new congestion to Kenmore because the loop splits before B/UR and C/D merge onto the 2-track Central Subway mainline. But it does mean that--however much you throttle up/down service levels on that little wraparound--you can guarantee those 66'ers a waiting train to Longwood on nearly every slot.
^Very useful indeed^. This is but one of many things having more interconnects enables.