Demolition on the first of three Somerville bridges being rebuilt for the Massachusetts Bay Transportation Authority’s $2.3-billion Green Line Extension was slated to begin March 23.

Boston transportation officials on May 7 unveiled an estimated $3.5-billion plan for modern Green Line trains that could potentially double the capacity of the nation’s oldest and most heavily traveled light-rail system.

An ongoing Boston Green Line track repair program has reduced derailments, but an aggressive maintenance program, improved safety standards and more investment are critical for avoiding more derailments and other system failures, according to a state transportation oversight report.

Jacobs Engineering Group’s intended purchase of CH2M, announced on Aug. 2, has apparently cost the latter its $57-million construction-management contract for the Massachusetts Bay Transportation Authority’s Green Line extension.