Despite its whimsical title, "Seoul Train" is deadly serious -- and yet so compelling that you can't stop watching even though you know it will haunt your dreams. Its subject is the "underground railroad" of North Korean refugees who are running for their lives in a desperate attempt to reach freedom. (On PBS's Independent Lens series, Tuesday, 10-11 p.m. ET. Check local listings.)

Getting out of North Korea, which this documentary accurately describes as the "world's largest prison camp," may be the easy part. Once they make it over the border into China, the refugees are hunted like rabbits by zealous Chinese cops and soldiers. Forcibly repatriated to North Korea, the refugees face torture and imprisonment for the treasonous act of leaving the country. It's a crime punishable by death. Some of the North Koreans interviewed for this film probably are dead already.

Apart from a few sickening scenes shot secretly in North Korea, most of the program takes place in China, where we meet groups of refugees awaiting rides on an underground route to safety. One of the most welcoming destinations is Mongolia, which has a reputation for treating North Koreans humanely before helping them reach their ultimate destination in democratic South Korea.

Schindler of Asia

We meet the first group of refugees as they plan a trip by train, taxi and foot across China to the Mongolian border. They include Han Sul-hee, who is 17. She and the rest of the group, mainly young adults who have left parents and siblings behind, are sitting in a safe house with a Christmas tree and Santa decorations. They have been waiting several months -- eating proper food and trying to gain enough weight so they'll look healthy enough to pass for South Korean tourists. So severe is North Korea's government-induced famine that the average 7-year-old child in North Korea is about half a foot shorter than his counterpart in South Korea, and it's estimated that up to three million souls have perished from hunger in recent years.

The camera follows Sul-hee and the others as they head for the train station in Yanji, China, for a journey that will be full of peril at every stage, especially in towns where the locals like to report foreigners to the police. The refugees' escort is Chun Ki-won, a South Korean pastor who has been called the "Schindler of Asia" for his rescue efforts. We last see him and his little tour group as they head into the Gobi, just a few miles from the crossing into Mongolia. The hidden camera could go no farther, so a message on our TV screen fills in the rest: Chun and all his charges were arrested at the border by Chinese police.