If you are coming from Beacon Park across the Grand Jct, you cross the
MBTA Fitchburg Route mainline at grade (under McGrath Highway). This
"diamond" is part of SWIFT INTERLOCKING, which has mainline crossovers and controls access to and from the Grand Jct plus to the
4th Iron to FX and the west end of BET/Yard 14 Layover. There's also a
power switch to the Cut Tracks off in the inbound mainline (west side of
the mainline).
When the Downeaster headhead moves come from Beacon Park, they
usually do what is refered to by some as "The Swift Shuffle" at Swift.
In other words, the cross the Fitchburg Route, then make a shift
west out No.1 Track, then head east via the mainline to North Station.
They rarely come through Yard 14 on this return trip. Ironically, when
they go TO Southampton Street, they usually go Tower A into Yard 14,
out to Swift to the GJB.
Dave