Thirty-seven people were injured after a fuel tanker caught fire and the death toll was expected to rise, the government said.
At least 12 killed in accident at Salang tunnel Authorities said the railway would link the Afghan capital, Kabul, with its north.
Public Works Ministry spokesman Molvi Hamiddullah Misbah said 37 people were injured after a fuel tanker caught fire on Saturday.
While the blaze has been extinguished, Misba said on Sunday the death toll could rise. The cause of the accident remains unclear.
“When we entered the tunnel in the morning, we saw unidentifiable bodies because they were badly burned. Women, men and children were among them,” eyewitness Ajab Gul told Afghanistan’s Toro news agency.
The landmark tunnel is about 90 kilometers (56 miles) north of Kabul and serves as a vital link between the north and the south of the country.
The Ministry of Defense posted on social media that military helicopters were dispatched to the scene along with medical personnel and first responders.
Abdullah Afghanmar, a senior health official in Parwan province, said many of the dead included women and children whose bodies were badly burned. “Among the dead, it was difficult to discern who was male and who was female,” he told AFP.
The 2.6-kilometer (1.6-mile) Soviet tunnel was an engineering feat linking Kabul to northern Afghanistan, linking the Indian subcontinent to Central Asia via the treacherous Sarang Pass, one of the world’s highest mountain roads One, at an altitude of about 3,650 meters (12,000 feet).
The pass is often closed for days due to accidents, heavy snowfall and winter avalanches.
In 2010, an avalanche on the Salang Pass killed more than 150 people.