When commercial Metro services start between Howrah Maidan and Esplanade along the East-West corridor, the trains will run at around 70kmph, Metro officials said.

But while crossing the 520m stretch under the Hooghly riverbed, the trains will travel at around 40kmph for the first few days. Not because of safety reasons but to allow passengers a better look at the special light effects and fish paintings that adorn the tunnel.

Blue LED lights on both sides of the tunnel announce that a train is “entering the Hooghly”. The lights are meant to provide an immersive experience, creating an illusion of water surrounding the train. Illuminated images of fish, sharks and other marine creatures appear on both sides of the tunnel.

“There is no speed restriction on the trains for the 520m stretch (under the riverbed). But in the initial phase, the trains will run at 40kmph on that stretch because we want the passengers to have a (better) look at the design,” a Metro official said.

“We plan to introduce some audio effects, too, so that the passengers can have a unique experience while passing through the stretch.”

The six-coach trains plying the East-West corridor, built by the Bangalore-based public-sector unit BEML, are capable of clocking speeds of 80kmph. But the optimum speed for the 4.8km Howrah Maidan-Esplanade section is just over 70kmph, an official said.

On March 6, Prime Minister Narendra Modi launched three Metro links in and around Calcutta — between Howrah Maidan and Esplanade, New Garia and Ruby, and Taratala and Majerhat. Commercial services on all three routes will begin on March 15.

The East-West corridor (Green Line), which will link Salt Lake Sector V with the Howrah Maidan when complete, is already operational between Sector V and Sealdah.

The New Garia-Ruby link is a truncated section of the upcoming New Garia-Airport corridor (Orange Line). The Joka-Esplanade section (Purple Line), which was already operational between Joka and Taratala, will now be extended to Majerhat.

The Howrah Maidan-Esplanade section is expected to draw a “substantial” number of passengers as the underwater tunnel links the twin cities of Howrah and Calcutta. This section has four stations: Howrah Maidan, Howrah, Mahakaran and Esplanade.

During peak hours there will be a train every 12 minutes; and the rest of the time, one every 15 minutes.

“A train will take around 12 minutes to complete one leg of the journey. At each station, it will stop for 20-30 seconds,” a Metro official said.

A train will make both the onward and return journeys along the same underwater tunnel, using either of the two tunnels.

“Since the full section has not been commissioned yet (the link between Esplanade and Sealdah is being built), the trains cannot complete one leg of the trip and then switch tracks for the return journey. They will make both journeys on the same track,” the official said.

For now, two rakes will be used for the commercial runs between Howrah Maidan and Esplanade.

“Since there is no provision for the trains to switch tracks, we cannot have more than one train on each track simultaneously,” the official said.