At least 50 flights were delayed and take-offs put off for four hours at the Indira Gandhi International airport in New Delhi as blinding fog enveloped the capital on Wednesday morning. For four hours till 9:30 AM, no flights took-off from the airport though planes were allowed to land using CAT-IIIB instrument landing system. Two flights were diverted. Fog started to descend at the airport in the wee hours due to which the runway visibility dipped below 100 metre and the general visibility to less than 50 metre.An airport official said, flights arriving at the airport were landing using CAT-IIIB instrument landing system. Two Jet Air flights arriving between 4 AM and 5 AM were diverted due to operational reasons. Sources said, the pilots were not trained to operate CAT-IIIB landing system. Due to poor visibility at the new runway, it was not operational and only the main runway was functional. Over 35 trains were delayed due to dense fog over Delhi and parts of North India. A Northern Railway spokesperson said the trains, which recorded disruptions due to fog included the Rajdhani Express trains from Patna, Howrah, Mumbai and Bhubaneshwar. These trains are running over two hours behind schedule while the other mail and express trains are reported to be running four to six hours behind their scheduled time. Northern Railways had earlier cancelled around 30 trains anticipating foggy conditions during winter. Dense fog in north India had hit train movement with 16 trains running late yesterday and 39 others cancelled. A senior Northern Railway official said, the Railway website is constantly being updated and the railway inquiry system is also being strengthened to inform passengers about the train position.