When you see a trolley with the single pole (literally) on top of the car, the pole is being forced against the wire with 25 pounds of pressure, created with large springs at the base. On the end of the pole is a U-shaped shoe that rides along the wire. Every now and then the trolley shoe will leave the wire, although many cars carry spring-loaded rope spools that yank the pole down against the roof to keep it from catching in the overhead and ripping down any wire.
In the case of the single pole trolley, the show has a carbon insert in it. As the carbon slides along the copper wire, it actually lubricates the wire and slides very easily. Catenary I believe is similar. A pantograph may heat up, but not enough to melt the wire. Wire for a high speed line is also much, much thicker than the wire used for a single pole trolley (a little thinner in diameter than a standard pencil.) I don’t remember what material the pantograph is made of, but metal on metal contact doesn’t always create a lot of fricton. Anyone else want to share any info on pantograph composition?