Many railway companies in Switzerland
Switzerland has its Swiss Federal Railways (SBB), but they do not service the whole country. There are many other railway companies as well. For a traveler, it is not relevant to know which company services which route. All companies work together and form the rail network of Switzerland.
On most routes there's only one company operating the trains. So there isn't really a choice. You just travel with the company that happens to service the route. There is no price difference on routes that are serviced by multiple operators. Tickets are purchased for a route, and are valid for any train. Rail passes are valid for either the whole country or a region, and are valid for all trains on the included routes.
The main railway companies
There are too many companies to list here, but the major ones can be found below: