Girder Rail
Hi TEP,
Like pgandw said, Girder Rail forms a flangeway for the trolley wheels.
But, Girder Rail isn't always used on tangent (Straight) track. The Flangeways can be formed with cobble stones, concrete, asphalt and other materials.
The Girder Rails are most important in turnouts and tight curves. It acts as a built-in guard or check rail to hold the wheel flange and keep the trolley from derailing.
Prototype railroads rarely used a turnout with a lower frog # than 6. And this was for yards and seldom used sidings. For mainline traffic, the frog #'s were much higher.
For Traction/Trolley lines, the frogs could be #4 and still allow a Trolley to go through them quickly because the Girder Rail helped to keep the wheels on the rails.
Check out the Proto87 Webpage Video to see what Girder Rail can do!