(CBS) — Thefts from a South Side rail yard — and in particular, the thefts of guns — have the attention of U.S. Sen. Dick Durbin.

Durbin, in a letter dated Tuesday, wrote Norfolk Southern Corp. CEO James Squires to say the railway giant must do more to secure the yard, which parallels the Chicago Skyway for several miles.

While Durbin writes in his letter that he is concerned about “any potentially dangerous freight shipments,” he says his biggest concern is the theft of guns.

He writes that the thefts “jeopardize the safety and well-being of Chicago residents.”

Three major gun thefts have occurred at various points in the yard since 2014. No one is suggesting that the thefts were inside jobs; rather, it is luck of the draw in a much larger ongoing series of thefts. Thieves took 30 guns in the latest such heist in which firearms were stolen, in September.

In 2014, 13 semi-automatic rifles were among the weapons stolen, while the following year 111 new guns vanished from a boxcar, at least one of which was subsequently used in a shooting. Durbin said only 16 of the guns taken in the heists have been recovered.

Durbin wants Squires to provide information on any plans to beef up security. He said he and his staff would be willing to meet privately with railway officials, so as not to jeopardize the measures.

WBBM has asked the railroad for comment. Earlier this month, a spokesperson told the Associated Press that it was doing “everything within its power” to thwart thieves, including the use of stepped-up patrols and the use of trained dogs.