To be fair, the Belmont section does have to run parallel to the Fitchburg Commuter Rail tracks. Having to deal directly with the MBTA likely slows things down.

IDK why they separated the project into 2 phases and are taking 4 years to design it though. It's not really rocket science: keep people away from active trains, add some crosswalks with flashing lights (HAWKs if you want to get fancy) and call it a day. Way safer than the existing routes to get out there and way more capacity since sidewalk riding is extremely limited.