Every rail line in Norway has a route number. The Flam Railway isn't a private tourist railway- although most of the traffic is now tourists (NiN and cruise ships), it is one of the public railways of the country.
All Flamsbana trains convey local people as well as tourists.
There are several wayside stations on the route which serve the locals.
Thus R45 is just the route number of the railway. Many, if not most of the public railways in Norway also have names (like the Flamsbana) as well as their route numbers.
Norway's best is just a re-seller, selling mainly tourist packages so they either want to sell you a return ticket or a single ticket as part of the Norway in a Nutshell package (both at enhanced price)
Vy-no is the right place to buy a single ticket for the line at face value.
The line was originally built to replace the old horse track down to Flam, but was only actually completed by the German Army of Occupation during WW2.
PS- Also book this as a through ticket from Flam to Bergen, not as separate tickets.