Does it make a difference to the stockyard or Union Pacific if the trains are robbed? If they're getting paid by the car it doesn't matter whether they're carrying goods or trash. (I genuinely don't know, just thinking why they might not have much incentive to protect the goods). Anyway the guards would have to be willing to tangle with well armed gangs and may not want to do so over Amazon packages, at least not at guard salaries. You'd need people who are into that sort of thing, and those people cost money/are more likely to shoot for fun, which could lead to bad publicity (I doubt that many people really care about the fate of whoever's busting into the cars, but there's obviously a bunch of people who come up behind as trash pickers and might be "target practice" to this type, horrifying a good slice of the rest of LA).