Technically, it is legal for someone to photograph pretty much ANYTHING from a public street or other property. It is also legal for Union Pacific or anyone else to question a photographer on a public street near their property, but they can't really enforce their wishes legally unless you're trespassing. If a firm doesn't want you photographing their property for trade-secret or security reasons, they should (and often do) build fences or move their operation far behind their property line. Top-secret government installations/operations aren't usually visible from public streets, even if public streets run close to those properties (Lawrence Livermore lab comes to mind as an example of that)