Calcutta: The tunnels of East-West Metro will have signboards to let passengers know when they are passing under the Hooghly or a heritage structure in the central business district.

Those who will miss the signage will get to know their coordinates from announcements on the train.

"These are the first transportation tunnels under a river in India and we want passengers to know when they are passing under it. Signage is one way of letting them know and announcements on the train are another," a senior railway official said on Sunday.

"There are likely to be similar announcements and signage when the trains pass under heritage or iconic structures such as the Writers' Buildings or St. Andrew's Church."

Officials of Kolkata Metro Rail Corporation (KMRC), the implementing agency of the project, have said the trains will start running on the entire route - between Salt lake Sector V and Howrah Maidan - by December 2020.

According to the official, the sign boards will be big enough for passengers to see them from inside a moving train.

Afcons, the private company that is boring the twin tunnels, has put small signage at the designated spots. "They will be replaced with bigger and illuminated boards before the service starts," the railway official said.

An Afcons official said the visitors who were allowed inside the tunnels were excited when they were told that they were under the Hooghly. "They were also keen to know at which point they were under an iconic structure," he said. "Among the visitors who had entered the tunnels - the top of which is 13m under the river bed - on a man-rider were engineers, railway doctors, lawyers, bureaucrats and ministers."

The twin tunnels, the boring of which started from Howrah Maidan, touched the diaphragm wall of New Mahakaran station a few days ago.

KMRC officials said around 6 lakh passengers would initially ride East-West Metro. By 2035, the count is expected to go up to 10 lakh.