The Franklin Avenue Shuttle subway line in Brooklyn is scheduled to close tonight at midnight for an 18-month, $74 million rehabilitation. Free buses will run the route in lieu of the subway trains. Renovation of the shuttle was won by Brooklyn elected officials, the communities near the line and transit activists after the MTA tried to remove the line from its capital program in late 1994. Despite its advanced state of decay, 10,000 riders per day have used the shuttle in recent years.