
The bus, carrying more than 50 people including the driver, was struck as it went over a level crossing near the small town of Marhanets in eastern Ukraine at around 9 a.m., the emergencies ministry said.
Ukrainian Transport Minister Kostyantyn Yefimenko was quoted by a news agency as saying the driver, who was killed, had driven forward onto the track even though a siren was signaling an oncoming train.
The bus was thrust 30 metres (100 feet) along the track by the impact and its hulk remained crumpled and buckled under the front of the locomotive for hours after the accident.
The emergencies ministry said 43 people, three of them boys aged between 7 and 15, had died. Nine people were in hospital with critical injuries and officials said the toll could rise further.
Television footage showed the bodies of victims laid out under blankets alongside the track. President Viktor Yanukovich, who was in the region at the time, visited the disaster scene and later saw some of the survivors in a hospital.
In televised remarks, he said that Ukraine needed to strengthen its laws related to the responsibility of drivers of public transport. He declared Wednesday a day of mourning.
Yefimenko was quoted by Ukrainski Novini news agency as saying the driver had at first stopped the bus and got out.
Quoting survivors, he said the driver then got into the bus and drove "right in front of the locomotive".
"Surviving passengers said they signalled to the driver that alarm lights were flashing and that he should not go forward," said Yefimenko.
Traffic police said the accident was the worst on the roads in terms of victims since Ukraine became independent in 1991.
First Deputy Prime Minister Andriy Klyuev, who was appointed to head an investigation into the tragedy, issued an order for all railway crossings in the country to be upgraded.
"A tragic event occurred this morning ... According to preliminary data, about 40 people have been killed," Prime Minister Mykola Azarov said at the start of a cabinet meeting.
Ukrainian police said in a statement that the bus was travelling from a hospital in Marhanets to a nearby town, Gorodishche.
Thirty-eight people were killed immediately. The others died in hospital or on the way there.
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