After working in the "freight" business for35 years with a common carrier, I understand the questions that were being asked in these letters. Although most here on MM see this as "pressure" on these carriers, the concerns of these Senators are very real. My guess is that if these letters are answered, guidelines will be formed for more secure shipment methods for firearms and related items with common carriers.
As a whole, security with common carriers is poor. My company routinely and high-end freight stolen. From TV's to candy to tires to small arms ammo, from single cartons to pallets to whole trailers. I used to know the numbers off the top of my head on what theft cost my company annually, but it was a lot. We had some items, like electronics, that we refused to haul anymore due to constant theft. We did haul firearms on occasion and yes, we did keep a little closer eye on those shipment, but for the most part those shipments were out there with the rest of the freight. Sadly, most of the theft came from "inside" sources. Even though the company thoroughly vetted potential employees, a few bad apples slipped through. Their biggest problem was hiring outside security companies. Minimum wage guards guarding millions of dollars' worth of freight. Well, you get the picture there.
The bottom line here IMO, is that the common carriers see new regs coming down the line to increase the security on shipments of firearms and related material, common carriers HATE "special" freight, add to that the possibility of liability issues, and with the small profit margins on these shipments, it's not going to pay to haul this freight. I know with my company, if they weren't making money on the freight, they wouldn't haul it. In the future you will see "dedicated" carriers for firearms that will have to meet certain requirements from ATF to transport firearms. Cost will go up, but it will take some guns out of the wrong hands.