The Massachusetts Department of Transportation's board of directors was to vote Wednesday afternoon to authorize a contract with Changchun Railway Vehicles to build 284 new subway cars for the Massachusetts Bay Transportation Authority. Changchun Railway Vehicles is a subsidiary of China CNR Corporation Limited, the Chinese government-owned enterprise that is the largest rail car maker in the world. The rail cars, the first of which will be delivered in 2018, are expected to help increase the subway system's capacity and decrease passenger wait times.