The corridor is about a 100 feet wide. 50 feet can be used for double track rail (as demonstrated by the width at some section on the Caltrain line). 40 feet is needed for a two lane busway and a trail (as demonstrated by the width on the BRT line and trail in Fort Collins, Colorado).
The Sodo busway in Seattle, which features a two lane busway, two track light rail, and a trail, is mostly less then 90 feet wide in total.
Planners claimed a need for a 65 feet right of way for rail citing an AREMA recommendation for a crash zone. This recommendation is used by freight railroads to demand a greater track distance between freight rail lines and passenger lines.
In some parts of the Dumbarton rail corridor, there’s class I trail adjacent or nearby that would negate the need for a trail in that section. In other parts, parallel streets make the busway on the right of way unnecessary.
In the case of Fort Collins, certain sections of the busway is narrower with only a single lane. Bus traffic in both directions are controlled by traffic signals.