Calcutta: The inauguration of the 16.5km East-West Metro line has been split into four phases instead of the originally planned two because of technical challenges emerging from each delay in implementing the project.

Railway officials said trains would first run on the Sector V-Stadium stretch and then till Phoolbagan, followed by Sealdah and, lastly, Howrah Maidan.

Metro highlights why and how the 16.5km route will be unveiled in four phases.

Phase I

Sector V to Stadium

Deadline: June 2018

It had been initially decided that trains would first run till Phoolbagan, the only underground station in the first phase of the project. Officials of the Kolkata Metro Rail Corporation cited two reasons for omitting Phoolbagan station from the inaugural route.

"The ventilation system at Phoolbagan station isn't ready. Some other infrastructure such as signalling and platform screen doors, escalators and elevators have also yet to be installed," said a railway official.

Phoolbagan not having a "crossover" for trains to switch direction clinched the decision. If the original plan were to be followed, a train would need to travel an additional 750m to reach Sealdah, which has that facility, and return all the way without any passengers.

Phase II

Stadium to Phoolbagan

Deadline: Early 2019

Although Phoolbagan station should be ready by then, trains would still need to travel an additional 1,500 metres to Sealdah and back just to change direction. But railway officials said keeping a fully-loaded station idle was not an option.

Phase III

Phoolbagan to Sealdah

Deadline: End-2019

Sealdah will be the key station for the East-West Metro with an estimated footfall of more than four lakh every day. Railway officials said a traffic survey had shown that many commuters who arrive at Sealdah every day travel onward to Salt Lake and Sector V.

Construction of the Metro hub at Sealdah is dependent on how soon the two tunnel-boring machines arrive from Esplanade. The machines have yet to start their journey.

"Once the tunnel-boring machines arrive, we will need another six months to wrap up," an engineer said.

The plan was to make Sealdah operational along with the rest of the stretch until a survey of passenger volume forced the change.

Phase IV

Sealdah to Howrah Maidan

Deadline: End-2020

Whether the rest of the underground stretch, including the tunnels under the Hooghly, will be opened as scheduled depends on the other sections being completed on time.