Fitchburg line: why single track through Waltham?

I had been wondering that for a long time especially since there is a Triangular signal for the eastbound trains near the Newton Street bridge, I always wondered why the single track which is only about a mile give or take and starts around Waltham Lumber and ends just about at the Waltham Center Outbound stop. You would think that the Jackson Street Bridge would have been removed years ago but it keeps on living with a wood and asphalt deck

from what I've read, the B&M needed more vertical? clearance for freight under the bridge, and by putting the track in the middle it solved the problem. The B&A had traditional wooden decked bridges with asphalt over them in Wellesley through the 1980s and they seemed well built, but the B&M infrastructure seemed a lot more rickety.

Adding to what BandA posted, the Fitchburg line always seemed lower priority until the Wachusett Extension.
Dropping the ROW might be the way to provide adequate clearance, especially if there's any possibility of electrification, because roadway approaches to the OHB are already probably maxed out for local traffic.

What is OHB? Waltham is Zone 2, so the fare is about 3X the Rapid-Transit fare for the same distance, and about 1.5X the Express Bus fare. It's just not fair.

Electrification can be accomplished using batteries to supply power to gaps, such as low bridges. But electrification of the Fitchburg should be a low priority.

More important would be restoring the Central Mass Line service to Clinton, with connection to the Fitchburg at 128 replacing inner portion. That would still leave a crying need for circumferential service along 128, and some sort of replacement for the Lexington Branch. Imagine some type of line running along, above or below 128 from Needham to Anderson via Riverside, Waltham, Lexington, Burlington, and Woburn?

OHB = overhead bridge (in relation to the railroad ROW).