Like most commuter railroads in the Northeastern United States, MBTA is a member of the Northeast Operating Rules Advisory Committee (NORAC) and uses the rulebook promulgated by that organization. Much of the MBTA Commuter Rail system is governed by NORAC rule 251, as the tracks are signalled for movement in one direction of travel only. During the 1990s, parts of the system, such as the Framingham/Worcester Line, were re-signalled to allow a more advanced mode of operations known as NORAC rule 261, which allows trains to operate in either direction on both tracks where double track is available. During the morning rush hour, both tracks can be simultaneously used for inbound traffic, allowing one train to make local stops while an express train overtakes the local train.