Why is the Lowell on an embankment?

I'm just curious as to why the Lowell Line is the way it is right now in the BET area. My assumption is that the yards in the area must have been gone by the time this embankment was built, and that all railroads were consolidated under B&M. So why didn't the grade begin to descend after Washington St and end in a junction with the Fitchburg near Lechmere? Or why doesn't such a reconstruction go hand-in-hand with the GLX? Seeing as the GLX will already require certain disruptions to the Lowell for track realignments and the rail-over-rail bridge by the BET entrance will need to be replaced at some point. So the way I see it, is: better spacing for development in the Inner Belt area, less "dead" space caused by a spaghetti bowl of tracks, and no need to replace a very old bridge later on.

Also, why did the Cobble Hill Track even survive, even if it is unused? There's already a connection from the Lowell to the Haverhill/Eastern lines.

My mistake... I figured I should check Historic Aerials to see if I could see anything and it turns out the embankment does predate the removal of the "Inner Belt" yard tracks. Though I'm still not sure why a route down to the Fitchburg never happened instead.

If you can ever get access to to the B&M employee publications of the late 20s, there is coverage before, during, and after the reorganization and reconstruction of the Boston terminal yards under George Hannauer. There may be some issues in the the B&MRRHS archives.

The Lowell Line is on an embankment because it was shifted in the 1920s during a big B & M consolidation and upgrade of Boston Terminal facilities. The Boston & Lowell was Boston's first railroad from the North and its several Boston terminal stations were about on the site of the O'Neill Building. (It is not by chance that some of the remaining streets in the area are named after stations or points on the B & L system - Billerica, Lowell, Lancaster, Merrimac, Nashua Sts.) The B & L made a direct beeline for points north, on the ground originally. After the Fitchburg got into the picture, the B & L was raised above the Fitchburg. This crossing was where the so-called "Red Bridge" was located, just west of Tower H. "Red Bridge" was removed a few years ago. The Grand Junction RR later also occupied space under it. In the 1920s, the B & M began planning major terminal improvements and the Boston & Lowell route was relocated to a new alignment from Mystic Junction down to about where the (New) Boston Engine Terminal was constructed and then paralleled the two Portland Division routes into North Station. This new routing was called the High Line. Yard 8, inbound freight, was constructed pretty much where the Boston & Lowell mainline between Mystic Jct. & Red Bridge had been. South of Red Bridge, the original Lowell route became a part of Yard 7. Yard 9 (where those commerical warehouses on Inner Belt Road are located) which was on the other side of the High Line from Yard 8 was excavated from a good sized hill located there. It was McLean Asylum Hill and the Asylum moved to Belmont at this time. The Lowell line is probably on part of the hill that was left, rather than any fill that was dumped there. This was quite an exciting time for the B & M with the construction of 1928 North Station, 150 Causeway Street, Hotel Madison (initially Manger Hotel), new Draw 1, Tower A, Boston Engine Terminal, new layout for Boston Yards, 130/131# rail on Fitchburg & Portland routes, CTC Rollinsford to Portland, T-1 engines, stone ballast, on mainlines, beginning of the filling of the Millers River and more. All within just a few years.

Speaking of the embankment, when will the new bridges be cut in by BET?