I just got off the phone with my neighbor across the street who is a USPS employee. She said that penalty for private use messages on envelopes is "between the company or government agency and the user of the envelope, whether employee or otherwise". She said the postal employees make no determination as to whether such an envelope is being used for personal use or not. As long as there is postage on the envelope or the company pays for it, they are happy.
So - wt1, your comment
It seems to me the railroad example leaves open the idea that one may use their envelopes for mail, as long as a stamp is affixed to pay the required postage, which is much more lenient than the Penalty Mail warning which prohibits use of the envelopes for anything but official government business
... I totally agree with you. This sounds like the railroad is saying that one can use the railroad's envelopes for personal use so long as the user pays the postage.
My neighbor also stated that if you reuse a business envelope that has a bar code on the bottom, be sure to cross it out. It has the original sender information and the envelope could be returned to that location!