Just realized something that the GLX won't be able to replace:
A single early morning outbound trip from Lechmere on the 80 bus and 87 bus, on Weekdays and Saturdays.
These 2 trips depart from Lechmere, bound for Arlington at 5:28 a.m. and 5:29 a.m. Monday thru Saturday; roughly following the GLX's Union Square and Medford Branches, respectively.
However, the first Union Square train doesn't arrive at Lechmere until 5:38 a.m., and the Medford Branch train until 5:45 a.m. By that point, the first 80 bus has already arrived at Medford/Tufts, once the first northbound E train reaches Lechmere station.
This means there will be a slight reduction in early morning northbound service with the GLX, or that transit riders who desire early morning northbound service, will still need to rely on the 80 and 87 buses respectively.
Technically, this problem already exists on the E branch on Huntington Ave., where the 39 runs earlier morning inbound service to Back Bay, than the E branch running inbound from Heath St.