Inside, the masses huddled for warmth. There was Marian Jiménez, who had sprained her foot. Jeremy Hidalgo, who had been walking four days.

Roberto Javier Tovar, who had left his wife and child behind in Venezuela, pulled in his jacket and praised the driver loudly, even though no one knew where the vehicle would take them.

“Almost no one has helped us but this man,” he said.

The sun began to settle and the back of the flatbed grew crowded as the driver took on dozens more migrants.

By evening, more than 100 adults and children were packed inside, leaving a silent, empty road behind.

“We must give praise to God Almighty for this blessing,” someone shouted when the vehicle stopped.

Night fell and stars came out. The temperature fell, too, but the truck was at last descending and the lights of Bucaramanga were visible, still thousands of feet below.

Daniel Bermúdez, who had left his family behind and had been walking for the last five days, looked out at the unknown city.

“My 6-year-old son, he saw me with my suitcase, and he said, ‘You’re not coming back,’” Mr. Bermúdez said as he began to cry in the icy wind.

He paused. “Yes, I’ll come back. But look at me now, I am so far from home.”