HEBBRONVILLE, TEX., JULY 9 -- Summer in Jim Hogg County was crawling by in its usual hot, humid buzz of insects Wednesday morning when John Cook saw the freight pulling in from Laredo. He was short-handed again, but he managed to round up three other Border Patrol officers for their daily check.

Cook, a short, friendly former combat medic in Vietnam and senior patrol agent here, drove the few blocks down to the Penitas crossing, pulled the four-wheel-drive vehicle up to the trackside deep in weeds, mesquite and sunflowers and helped capture three Mexican men who jumped off one rail car and tried to escape.

Methodically, he and his men split up to inspect the train from each end toward the middle. The last car they had time to check before the train departed was a closed metal trailer riding piggyback on a flat car. They cracked open an outside lock to reveal -- sprawled on the floor, shirts and shoes discarded, gasping for breath -- 19 more Mexican men.

The near tragedy, in the minds of the agents here, is remarkable because it is so unremarkable. Once or twice a week, they find undocumented immigrants trapped in hot, nearly airless containers of one sort or another. A few deaths each year are expected -- although the 18 found suffocated last week in a boxcar in Sierra Blanca went far beyond anything experienced here.

After a drop in the number of illegal border crossings early this year in the wake of a stricter immigration law passed last November, the Immigration and Naturalization Service reported this week that the number is climbing again. Given the illegal aliens' numbers and energy, and the difficulty of rail patrols, many Border Patrol agents express surprise that there are not more tragedies.

The deaths at Sierra Blanca and the near deaths here have raised familiar outcries from all sides of the debate over immigration reform. Opponents of new restrictions on hiring immigrants say such dangerous border crossings should be unnecessary when aliens do so much essential, low-paid work in America.

Supporters say such tragedies will not end until sanctions against employers who hire illegal aliens convince young Mexicans there are no longer American jobs for which they need risk their lives.

J.J. Fulgham, assistant chief patrol agent for the Laredo sector, said the Border Patrol's ability to catch aliens as they ford the river at Laredo has increased with the advent of electronic sensors and low-light television monitors.

Agents on foot and horseback check the riverside and the freightyards next to the river just before the trains are due to leave. Agents at smaller patrol outposts like this one check again further down the line.

Despite technology and a substantial boost recently in authorized Border Patrol manpower, they cannot catch everyone, and the peculiar logistics of checking railroad cars make matters worse.

Richard Estrada, research and publications director for the Federation for American Immigration Reform, spent much of his youth in the border city of El Paso. He noted that inspecting rail cars is physically more taxing than manning the automobile roadblocks found on all major roads out of Laredo, El Paso and other border towns.

The roadside checkpoints have air-conditioned trailers where agents can take a break. Rail cars must be checked on foot, sometimes on the run, in the often brutal south Texas heat.

Although the Hebbronville Border Patrol station has an authorized strength of 35, only 17 agents work here now. Little net gain is expected because so many of the officers here are seeking transfers.

"There is very little here. There are no good schools," said Cook, who has lived here 2 1/2 years with his wife and two children. "I've been trying to get transferred myself."

Illegal immigrants know the agents have trouble checking all the cars. Riding the rails, despite its dangers, is often cheaper than catching a bus or paying a truck driver to smuggle them to San Antonio. Even when Cook and his men flush them from a freight, they can often hide in the mesquite until the train pulls away, then climb back on. They are called the "grubbing hoe gangs," for the sharp-ended gardening tools they sometimes carry to wedge their way out of rail cars.

The 80-car Texas Mexican Railway freight from Laredo Wednesday was bound for Corpus Christi, with the trailer containing the 19 men set to go all the way to Houston. It was one of two long freights that leave Laredo, on the Rio Grande border, every day. The other freight, usually longer, heads north to San Antonio.

After more than an hour in the locked trailer, 23-year-old Pedro Palma said later, the temperature soared above 125 degrees and men were awash in sweat.

"I was having a lot of trouble breathing," he said this morning, just before Border Patrol agents returned all 19 men to Mexico. "People took off their shoes and shirts and then waved them in the air to try to create a breeze."

When the train stopped here, some inside whispered frantically to quit the fanning before the noise was detected. But Palma, who thought he might die in the trailer, was grateful when the agents forced open the doors.

Cook put his subdued, cooperative prisoners into two vehicles. The cars were so overloaded their bottoms nearly scraped the road, but he was glad all aboard were alive. Border patrol agents expect that many of the men rescued Wednesday will try to return soon, as long as some can find jobs. "We just give them very mixed signals," Estrada said.