A Georgian official said the train hit a land mine and blamed the explosion on Russian forces, who withdrew from the area Friday. The Russian Defense Ministry declined to comment.
The blast came amid persistent tensions in Georgia. Russia pulled the bulk of its troops and tanks from its small southern neighbor Friday after a brief but intense war, but built up its forces in and around two separatist regions - South Ossetia and Abkhazia - and left other military posts deep inside Georgia.
The guided missile destroyer USS McFaul, loaded with 72 pallets of humanitarian aid, is the first of five American ships scheduled to arrive this week.
The much-needed aid and the damaged train were a stark reminder that it will take substantial amounts of aid and many months of rebuilding before Georgia can recover from the war with Russia. Five days of fighting damaged cities and towns across the country and displaced tens of thousands of Georgians.
West of Gori, an Associated Press reporter saw 12 derailed tanker cars, some askew on the railway line and others flipped onto their sides. Firefighters hosed down the wreckage.
Georgian Interior Ministry spokesman Shota Utiashvili said the train hit a mine. He said there were no casualties, but the blast had also set off explosions at an abandoned munitions dump nearby.
Utiashvili blamed the incident on the Russians. Georgian officials say Russian forces have deliberately damaged infrastructure to weaken Georgia, and accused them of blowing up a train bridge last week.
Georgian Public Television, which showed massive clouds of billowing black smoke rising from the fire, said another 20 train cars had been uncoupled and pulled away to prevent the fire from spreading.
The director of Georgia's railways, Irakli Ezugbaia, agreed the blast was probably caused by a mine. He said an investigation was under way and other mines had been found on the tracks.
Georgian forces removed a large artillery shell that had been jammed under the tracks and covered with stones.
Ezugbaia said the train was carrying crude oil from Kazakhstan to a Georgian Black Sea port.
Georgia straddles a key westward route for oil from Azerbaijan and other Caspian Sea nations including Kazakhstan, giving it added strategic importance as the U.S. and the European Union seek to decrease Russia's dominance of oil and gas exports from the former Soviet Union.
The conflict between Russia and Georgia, a small ex-Soviet republic whose pro-Western leaders have tried to shed Moscow's influence and sought NATO membership, has brought Russian-U.S. relations to a post-Cold War low.
A U.S. official said the American ship anchored in Batumi, Georgia's main oil port on the Black Sea, because of concerns about the state of Georgian port of Poti.
Russian troops still hold positions near Poti, and AP journalists there have reported on Russians looting the area. Georgian port officials say radar, Coast Guard ships and other port facilities were extensively damaged by Russian troops.
At dockside in Batumi, with the McFaul anchored offshore, U.S. Navy officials in crisp white uniforms were met Sunday by Georgian officials, including Defense Minister David Kezerashvili.
Local children gave the Americans wine and flowers.
Speaking to The Associated Press on the aft missile deck of the McFaul, anchored a mile offshore, Kezerashvili said that "the population of Georgia will feel more safe from today from the Russian aggression."
"They will feel safe not because the destroyer is here but because they will feel they are not alone facing the Russian aggression," he said.
The commander of the five-ship U.S. task force, Navy Capt. John Moore, downplayed the significance of a destroyer bringing aid. "We really are here on a humanitarian mission," he said.
The McFaul, an Arleigh Burke-class destroyer, is also outfitted with an array of weaponry, including Tomahawk cruise missiles, which can carry either conventional or nuclear warheads, and a sophisticated radar system. For security reasons the Navy does not say if ships are carrying nuclear weapons, but they usually do not.
The deputy chief of Russia's general staff suggested that the arrival of the McFaul and other NATO members ships would increase tensions in the Black Sea. Russia shares the sea with NATO members Turkey, Romania and Bulgaria as well as Georgia and Ukraine, whose pro-Western president also is leading a drive for NATO membership.
"I don't think such a buildup will foster the stabilization of the atmosphere in the region," Russia's ITAR-Tass news agency quoted Col. Gen. Anatoly Nogovitsyn as saying Saturday.
An Associated Press television cameraman and his Georgian driver were treated roughly and briefly detained by Russian troops outside Poti on Sunday as he shot video of Russian positions outside the port. A soldier smashed his microphone with a rifle butt, shoved him to the ground, then marched him to an armored vehicle and detained him and his driver. They were later handed over to Georgian police, who released them.
Hundreds of Georgians flocked back to Gori on Saturday, one day after the Russians withdrew, to begin the Herculean task of rebuilding their lives. Their homecoming was laced with despair, disbelief and anger.
"Barbarians, that's what they are. They kill innocent people here ... how many kilometers (miles) outside the battlefield? They bombed all over Georgia," Zurab Gvarientashvili, a 31-year-old engineer, said as he viewed his apartment, destroyed by a Russian bomb.
Gori is 30 kilometers south of the capital of the separatist region South Ossetia, where Georgian forces launched an assault on Aug. 7, sparking the war and an international crisis.
South Ossetian officials accused Georgia on Sunday of building up military forces along the edge of South Ossetia and claimed a Georgian unit had fired sporadically at villages overnight. There were no reports of casualties, but South Ossetian spokeswoman Irina Gagloyeva said residents were asking to be evacuated.
Georgian Security Council chief Alexander Lomaia denied that Georgian forces had fired any shots but said existing agreements oblige Russian forces to leave positions in the area, which is in Georgia.
"We are waiting for the Russians to withdraw and to take the area under our control. We do not intend to do this by force," he said.
Lomaia also said Russian forces were still holding 12 of 22 Georgian servicemen taken prisoner in Poti last week.
Next to one bomb crater in Gori, Merdiko Peredze's goats grazed on burnt grass.
Peredze said he was refugee twice over - once after fleeing his home amid fighting in the early 1990s in Abkhazia and now again, with his house in Gori in tatters.
"I'm an old man but I will return to Abkhazia," he vowed. "Russian, Georgians, Ossetians - we should all be living in peace together, like we did under Stalin."