AS a monster freight train comes to a slow, moaning stop in the desert night, Border Patrol Agent David Johnson races from railcar to railcar checking for stowaways in the "knuckle" areas that hold the train together.Within 10 minutes, the train is on its way again and Mr. Johnson and two other agents have collared two Mexican youths - one had climbed aboard 1,000 miles back in El Paso, the other had jumped on in Yuma, Ariz., and both were without documents. The railroads are one of the most popular - and dangerous - forms of transportation for undocumented aliens once they slip across the border, says Jose E. Garza, chief patrol agent in Laredo, Texas, and national coordinator of Operation Ironhorse, a nationwide series of intensive train sweeps by law-enforcement agencies and private railroad companies. In Operation Ironhorse II, covering 10 days in late June, 4,339 undocumented aliens were apprehended on trains nationwide, according to Border Patrol statistics. During that period, Mr. Garza says there were numerous accidents involving undocumented aliens, among them a Mexican woman who fell while trying to jump a train near Harlingen, Texas, and lost both legs; a Mexican boy who had the toes on one foot crushed; and in El Paso, seven undocumented aliens were found sealed in a land-sea container near de ath from lack of water and food, Garza says. Operation Ironhorse may only hint at the scope of the problem, considering the numbers of undocumented aliens that just a single rail company turns up in a given month. "We apprehend 4,000 to 5,000 [stowaways] a month," says Lou Butler, general director of police for the Union Pacific railroad. Practically all those arrests are made in the southern border region and most are undocumented aliens, he says.