They say there is a prototype for everything. Here is a rail joiner used to join 2 rails of different dimension. The photo was taken on the CNR Garneau Yard in Shawinigan Quebec.
This photo taken by a friend of mine is also posted with his permission.
On a more mundane note, blue is usually a symbol used to keep motive power out or off. I wonder if locomotives are forbidden to cross onto the lighter rail.
As for the transition or compromise joiner, I’m sure the real railroads were using those before model railroaders. The original method always shown in odl mazaines is to flatten the joiner on the smaller rail side and solder the base of the smaller rail on top of the joiner. The various brands of transition joiners for model track are all much newer.
These types of joiners often show up on a siding off of a main line for example. The turnout to the siding must be the same rail height as the main but the siding may be a lower weight rail as it does not have to put up with weights and speeds of the main.
So they often turn up at the first joint in the siding away from the turnout.
Not sure I understand the question. Railroads have long used different sizes of rail and needed a way to join them together. Transition rail joiners are probably almost as old as railroading itself and certainly not unusual.