I can tell you that the Windows license is the cheapest part of costs for such a thing.
"Why is it connected to the Internet."
Simply, because you want to have the correct and most current departure times with all delays displayed. These data has to come from somewhere.
"Why no Raspberry PI"
Because the Raspberry is nice for rapid development, but does not fulfill any of the requirements in terms of EMI, temperature rating, shock proof, reliability, and so.
Embedded world is different from your home environment, where you have a controlled temperature, almost no EMI problems, and stuff like this.
I am by no means a Windows advocate. I just want to point out, that embedded is a completely different area, which is the reason why there is not only Windows, Linux but also QNX, VxWorks, OS9 and others operating systems existing.
Kiosk systems are most likely Windows based for the reason, that HMI development is dirt easy, why most companies are doing it this way.