My thinking is that the New Hampshire Main will get doubled-up service by kicking the Haverhill trains off the Western Route, so Indigo would be the only service from Reading-south. Winston has mentioned issues with the single tracking from Oak Grove to Sullivan crimping the headways of a DMU service though, not sure if that changes if we get rid of Haverhill trains on that line.
The helper for the single-track pinch is this surface
freight track at Wellington for access to the Medford Branch and some long-gone Malden customers. Starts starts at Cabot Rd. just past the Route 16 overpass, stays on the surface when the other 3 OL and commuter rail tracks go through the tunnel, and has a crossover back to the mainline about 1/2 mile up. Track then continues for 1/4 mile further over the Medford St. bridge before starting down a fenced-off incline to an abandoned freight siding that used to cross Commercial St.
The crossover past the tunnel is out-of-service because that's Pan Am's maintenance responsibility and hasn't been used in years. But at minimum a refresh of the track around the tunnel with automatic switches installed on each end gets you a much-needed passing siding for not a lot of money. Going the full 3/4 mile to Medford St. would require shoring up the first 100 ft. of embankment where the ex- freight siding's
incline down starts right after the bridge, but also wouldn't be a lot of money.
Now...would this get you 15 min. DMU headways to Reading? I don't know. A traffic engineer would have to model the max capacity and train meets with one of those stat plots you find in those 50 pages of official study appendixes. Not a question we can speculate about with unless somebody here is a wiz with Minitab and wants to spend all day plugging data. My guess is that you can get close enough to 22 +/- 3 mins. clock-facing that it's full game for the Indigo spider map if all Haverhills were moved permanently to the Lowell Line and North Wilmington got swapped out for a reopened
Salem St. on the Wildcat Branch (the stop N. Will ultimately replaced, with station site still owned by the T). The only major consideration is that Eastern Route must still get full priority at the single-track pinch @ Sullivan Sq. used by all non-Lowell/Fitchburg trains, so Reading inbounds may have to pause frequently for schedule adjustment on the little piece of double track next to Assembly Sq. to let an Eastern Route or Reading outbound pass. On a very short line with no thru-running Haverhills, that's not a big deal and the fudge factor can be built into the schedule. All of the Indigos are going to have to bank a couple minutes' fudge factor into their schedules to let long-haul trains with priority pass.
This is specifically mentioned in some MPO report with a diagram (current ed. of LRTP?), but I can't for the life of me find it on Google today. I do not think the T has studied it in any published report, so if it's on the front-burner that's internal info. It would not have been studied by anyone yet in a Reading short-turn context, as any recent needs assessments for the line are centered around the full Haverhill schedule. And the full Haverhill schedule can't have any increases (not even on the new upstream double-track, which is mostly there to solve the line's wretched on-time performance) until they relocate out of tiny Bradford layover yard to more spacious digs. And even then the Lowell Line is where the most slots can be gained without spending a penny more. So nothing about this passing track truly matters until they seal the deal on a new layover parcel.
There's no question it's feasible and would eliminate some meets since that out-of-service crossover at the 1/2 mile mark is already there. The main question is whether the extra running space to just shy of Malden Ctr. reduces more meets and is straightforward enough on embankment + retaining wall work to go for, and looking at what it means for density of Reading turns vs. end-to-enders.
BTW...the other piece that has to be done is Reading station itself, extending the double-track 1/3 mile from Ash St. through the platform. But that has to be done for any/all service because the single platform is a bottleneck today. You can make a reliable assumption that some small sum of found money will appear fix this small detail in the next 3-5 years because it's consequential to on-time performance as well as service increases. So pay it no mind when pondering DMU's because it's the universal constant for anything/everything.