The extension was originally to continue further north to Reading and replace commuter rail service entirely between Boston and Reading. However, that extension was voted down by residents of Reading. Under that plan, without commuter tracks between Boston and Reading, Haverhill trains would have solely used the Lowell Line and Wildcat Branch - a routing used today only for a handful of rush hour trains and Amtrak's Downeaster service. A third express track was built between North Station and Wellington station which would have been extended north to provide express service to Reading. Due to the extension being cut back to Oak Grove, no regular service uses this third track.
Most of the stations are built in a Brutalist style, with angular concrete forms, except for the infill station at Assembly, which opened on 2 September, 2014.
From north to south, the stations added or rebuilt for the extension: