NEW DELHI: Prime Minister Narendra Modi on Tuesday inaugurated the 4.94-km-long Bogibeel Bridge in Assam , India’s longest rail-cum-road bridge, on the mighty Bramhaputra. PM Modi flagged off the first train from Tinsukia in south bank to Naharlagun in Arunachal Pradesh in north bank. Modi along with Assam governor Jagdish Mukhi and chief minister Sarbananda Sonowal also walked a few metres on the bridge. Here is all you need to know:* The bridge has been thrown open on the birth anniversary of former Prime Minister Atal Bihari Vajpayee under whose government the work had started in 2002. The foundation stone of the project was laid by former Prime Minister HD Deve Gowda in 1997.* The Bogibeel Bridge, which was a part of the Assam Accord and sanctioned in 1997-98, is likely to play a crucial role in defence movement along the India-China border in Arunachal Pradesh.* Constructed at an estimated cost of Rs 5,900 crore, the bridge has a "serviceable period of around 120 years". It is situated 17 km downstream of Dibrugarh city in Assam, and it has been constructed for double-line broad gauge track along with three-lane roads.* The road distance from Dibrugarh to Itanagar will be reduced by 150 km and the railway travel distance between these two points will shorten by 705 km, while this bridge will provide an alternate and shorter route from Dibrugarh through north bank of Brahmaputra to Delhi and Kolkata via Rangiya.* Although it will ease out inconvenience caused to people living on the northern side of Brahmaputra to a great extent, officials said the defence requirement played an important role while sanctioning the structure and its design.* The biggest advantage of the bridge will be easy movement of troops from southern to northern bank. This means travelling to the farthest most point of India's border with China will be shortened by several hundred kilometres.* This bridge will be the lifeline of the north eastern part of the country and will facilitate connectivity between north and south banks of river Brahmaputra in the eastern region of Assam and Arunachal Pradesh.* Students and patients have to cross the river on boat to come to Dibrugarh, which houses the famed Dibrugarh University and Assam Medical College, besides a dozen other such centres* The bridge will enhance the national security of the eastern region by facilitating swift movement of defence forces and their equipment. It was constructed in such a way that even a fighter jet can land on it in case of emergency.* Bogibeel is India’s first and only fully welded bridge construction. It is also for the first time European codes and welding standards were adhered to in the construction of a bridge in India.* Bogibeel Bridge falls in Seismic Zone-V, which is most vulnerable to earthquakes with magnitudes more than 7.0. To offer good stability, the heavy spans (1,700MT) are provided with seismic restrainers.* Realising its strategic importance, the central government had declared the construction of the bridge as a National Project in 2007, thereby assuring availability of fund for speedy construction.(With agency inputs)