There are over 37,000 railway bridges which are 100 years old with almost 32 per cent of them being in the Northern Zone, the government informed Parliament today.In a written reply in the Lok Sabha , Minister of State for Railways Rajen Gohain said there are 37,162 railway bridges in the network which are a century old."The age of a bridge does not have any direct relevance on the physical condition of the bridge. There is a well established system of inspection of bridges of the Indian Railways "All these bridges are inspected twice a year, one before the onset of monsoon and one detailed inspection after monsoon. In addition, certain bridges are also inspected more frequently depending on their condition," the minister said.The Northern Railway Zone has 8,691 such bridges, followed by the Central Zone with 4,710 bridges, Eastern Zone with 3,119, South Central Zone with 3,040 and Western Zone with 2,858 bridges, he said.Repair, strengthening, rehabilitation and rebuilding of railway bridges is undertaken whenever so warranted by their physical condition and if these measures take long, then safety measures like speed restrictions are imposed, the minister said.He said that during the last five years (2012-2013 to 2016-2017), a total of 3,675 bridges have been repaired or strengthened.As on April 1, 2017, Gohain said a total of 3,017 railway bridges are sanctioned for repair or rebuilt.