Metro-North's busy and aging New Haven line needs $3.6 billion in infrastructure work by 2020 to keep the rail running smoothly and in good repair, according to a study released Thursday.

The Regional Plan Association's report said the most expensive investment -- replacing or renovating five century-old movable bridges in Connecticut -- has a $2.8 billion funding gap. Other investments include an additional $330 million to expand the New Haven rail yard, $300 million for signal and communication systems work and $85 million for station and platform repairs.

Connecticut Department of Transportation spokesman Judd Everhart said the agency in recent years has invested about $2 billionfor new rail cards, the line's overhead catenary wire system, station improvements and maintenance at the rail yard.