Interpretation of "If x R.R.'s are owned"
The use of passive language here is quite annoying, and the official Monopoly rules from the Hasbro website don't actually provide enough detail to give a definitive answer. There are plenty of websites that argue in favour of the interpretation that it's based on how many Railroads are owned by the player collecting rent, but for a definitive answer? We have to go back.

In fact, the version of the rules that seems to most clearly explain how to collect rent on the railroads is the 1904 version of the Landlord's Game, by Lizzie J Magie, whose design was taken and rebranded as Monopoly (and Clarence Darrow credited as inventor) about 20 years later. In these rules:


  If one player owns 2 railroads, he charges $10 fare; if 3, he charges $20; 4, $50.


This is reflected in several versions of the rules available on the linked site. Unfortunately, I've found it hard to find exactly where the rules stopped bothering to explain this, but as far as I can tell this has always been the common interpretation (but Monopoly is famous for having rules that "everyone knows" even though they aren't official, so this doesn't necessarily mean much).