The Maritime Aquarium at Norwalk is set to undergo expansion, with a large seal exhibit and 4D movie theatre to open at the attraction in 2020.

The new seal habitat will be almost seven times the size of the existing 22,000 exhibit, which is part of a larger project that will also see a new 4D theatre built at the Connecticut aquarium. Construction has just started on the new facilities, which are set to open in late 2020.

These endeavours are an advance response by the aquarium for the need to rebuild a nearby 19th-century railroad bridge that keeps getting stuck open and disrupting rail services. The railroad bridge runs between the aquarium and its existing IMAX theatre, which, at six storeys high, is the largest in Connecticut.

Work on the bridge is expected to begin in 2021 and will require the removal of the IMAX theatre and a riverfront structure currently home to the Maritime Aquarium's Go Fish and Meerkat exhibits.

Aquarium bosses have negotiated a US$40m (€36.4m, £31.2m) agreement with state and federal officials to build the new theatre and fortify elements of the aquarium that will be impacted during bridgework.

"Minimising the significant economic damages that will occur due to the project is critical if the aquarium is to remain one of the state's largest tourist attractions and an economic driver for the city (of Norwalk) and state," said Dave Truedson, the aquarium's chief operating officer.

The new 150,000-gallon, two-storey, seal exhibit is to be built on the site of the harbour seals' current display, requiring the animals to be moved temporarily into the Go Fish exhibit, which at 30,000 gallons is also bigger than their previous habitat. It will also be the largest at the aquarium – 40,000 gallons larger than the shark-filled Ocean Beyond the Sound exhibit.

The Maritime Aquarium said the design of the tank is based on behavioural animal studies and on recommendations from member institutions of the Association of Zoos & Aquariums. Its greater depth and size will protect the seals from noise and vibration caused by the bridgework.

Simultaneously, the aquarium is working with SimEx-Iwerks Entertainment on building the new 179-seat 4D theatre on the site of the current employee parking lot. It will have 4D sensory seats, be capable of 2D and 3D digital screenings and have a stage for lectures and other guest presentations.

The Maritime Aquarium at Norwalk is a not-for-profit organisation whose mission is to educate visitors about Long Island Sound, allowing them to get close to 300 species native to the Sound and its watershed.