Eurostar scrambled to get tens of thousands of passengers home for the holidays as it resumed service of high-speed trains under the English Channel on Tuesday, four days after a service shutdown caused by a hallmark of Christmastime travel: snow.

The first train pulled out of Paris' Gare du Nord station in the morning carrying 750 passengers, many of whom had been stranded since the rail link between Britain, France and Belgium was suspended Saturday.

Hundreds of others waited in a line that stretched across the cavernous Paris train station, as Eurostar staff circulated with trays of pastries and coffee in paper cups.

Eurostar has identified the problem that caused trains to break down in the Channel Tunnel, sparking the shutdown _ unusually dry, powdery snow that got into the engines. The shutdown affected 40,000 people and left French President Nicolas Sarkozy indignant.

Eurostar offered its "deepest apologies," promised compensation and pledged to do its utmost to make sure passengers can celebrate the holidays with their families.

"We're worried about the passengers first and getting them where they need to be for the Christmas break," said Ian Nunn, Eurostar finance director. "We'll worry about reputational damage afterwards."

People who were supposed to travel over the weekend were given priority as trains started up. Service was less frequent than usual: About two out of three regularly scheduled Eurostars were running.

At the Paris station, Britain's Isabella Comba was worried.

"My mother's sick, so I really need to get home to be by her bedside," said Comba, who works at Sotheby's auction house and was hoping to catch an afternoon train to Cornwall, in southwestern England. "It will really throw off my plans if I'm not able to get to London today."

Eurostar's operations chief, Nicolas Petrovic, gave a detailed technical explanation of what happened. Normally snow in the region tends to be wet and heavy, he said, but unusually dry snow got past the train's snow-screens and into the engines Friday. Then the snow turned into condensation inside the Channel Tunnel, where temperatures were higher than those outside.

The condensation caused the trains' electrical circuits to fail, he said.

"It's the first time we have (had) these snow conditions in 15 years," he said. Eurostar has commissioned an independent review into the problems.

Nonetheless, Transport Minister Dominique de Bussereau expressed incredulity that mere snowfall had caught Eurostar off-guard. So did ordinary travelers.