The train's whistle roused Chen Xi from her sleep as her bed vibrated to the rhythm of the tracks.

It was 2 am and again she had been woken by the shrill, even frightening, sound.

Chen drew the curtains, trying to keep the chill wind from sneaking into her tiny room. She knew it would be another long, cold night, the same as the previous night and the night before.

Chen was in a berth on a four-carriage train, actually a mobile eye hospital. It travels to remote areas in China and provides free cataract operations.

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