New Delhi: Thirty-one trains were on Wednesday cancelled due to the dense fog that enveloped the northern region.

While departure timings of seven trains were rescheduled, 31 trains are running several hours late, a Northern Railway official.

West Bengal Sampark Kranti Express and Farakka Express are running 24 hours and 18 hours, respectively, behind the schedule.

Acknowledging inconvenience to passengers, the official said "safety of passenger is the main issue for us. So trains cannot be allowed to run in poor visibility condition."

Loco pilots are being instructed to run trains in slow speed of 20km to 30 km per hour in foggy weather.

Railways have also partially cancelled 11 trains due to the weather condition, the official said.