That's Saugus River Junction. It was actually built by the Eastern, not the BRB&L, in 1881 as the Chelsea Beach Branch: an Oak Island-Point of Pines loop branch to direct-compete with the narrow gauge BRB&L. What we now know to be 'the' BRB&L ROW through Point of Pines is actually the Eastern branch's original ROW, while the BRB&L's original ROW ran right smack on the beach north of Revere St. to the present-day Lynnway rotary. The Eastern's branch used Saugus River Jct. and its smaller drawbridge to rejoin the main and cross the river on big Saugus Draw, while the BRB&L came off the beach along Lynnway, swung very close by the Eastern's branch, then grabbed its 'traditional' drawbridge river crossing into Lynn (where the present-day pipeline crossing is next to Route 1A). The branch got heavy use under the Eastern with hourly local service, but only lasted 10 years before being mothballed by B&M soon after they bought out the Eastern in 1890. Losses from the head-on competition with BRB&L and the rise of electric trolleys made it fruitless for B&M to keep running such hyper-local Revere Beach service. But rather than scrap it B&M sold the branch lock, stock to BRB&L who relocated their tracks off the beach in 1896 onto the superior grading of the ex-Eastern branch. The BRB&L saw its biggest financial boom times right after the relocation, and was instrumental in helping them self-fund their electrification.
The original Chelsea Beach Branch wye with the Eastern main at Beach St. right by Wonderland was severed upon sale. NB leg well-traceable by the dog park's property lines, SB leg kinda-sorta traceable a couple generations-removed of parkway road construction by the pronounced S-curve VFW Pkwy. makes on the last couple blocks by the rotary. Saugus River Jct. stayed under adaptive reuse as a seldom-used interchange track between B&M and BRB&L for a few years, but was taken out when Saugus Draw's 19th century span got wholesale-replaced by the current 1911 span. Though the junction draw was demolished at the same time, the junction remained on topo maps until WWII as a 'phantom' rail line kept under OOS status with the ICC; BRB&L retained the bridge-less embankment as a future consideration for combining Lynn depots and main river crossings with B&M should they ever have wanted to explore that in the future. It wasn't expunged from maps until BRB&L went out of business outright in 1940 and that future consideration was no longer needed. So there's nothing real being depicted on the junction crossing on any topos after approx. 1910. All told, it lasted considerably less than 30 years as a railroad structure...and only one you could ride over yourself for about 8 or 9 of those years.
Today those same approach spans are used by the power lines that run alongside the Eastern Route as their means of crossing the river. And the various Blue Line-Lynn extension alignments that choose a Point of Pines routing (as opposed to a Wonderland or Oak Island Rd. hop-over) recycle Saugus River Jct. as the means of S-curving over to the Eastern ROW. All alignment considerations for BLX call for a 4-track taller fixed-span replacement for Saugus Draw carrying Blue and RR side-by-side, rather than doing a wholly duplicate reproduction of the old BRB&L main crossing. For just the PoP alignments, they'd recycle the well-kept Saugus River Jct. embankments and move the power line towers around to install a smaller fixed span carrying the Blue Line, and avoid the need to widen any of the Eastern ROW's 2-track embankment through the marsh.
