One of the things that kills rail in big cities is long periods of maintenance. People who are frequent railriders will just figure out new ways to commute to avoid the hassle of rail delays, single tracking, etc. Once those people get settled in their new commute pattern they rarely go back to riding rail.
It works both ways. Seattle recently closed down highway 99 in town to complete the tie-in of the new tunnel replacement for the Alaska Way Viaduct. An estimated 30,000 vehicles per day disappeared from the daily commute in favor of the, busses, train and light rail system. Now that the tunnel has opened, a large number of those cars have not returned to the road as the commuters have become used to the public transport systems.