The system was popular for trams in the late 19th/early 20th century; but most have now converted to pantographs. It is still the only way that a trolley bus system can take power - there just isn't enough room for 2 pantographs side by side.

The key advantage of trolley wire is that the position of OLE no longer needs to be strictly controlled relative to vehicle. This has obvious advantages for unguided trolleybuses, but is also useful for early trams, which used a dense network of tracks.

Removing the strict geometrical relationship between track and OLE meant more flexible wiring arrangements, with groups of wire runs clustered in the centre of tracks.

It is also much easier to mount a trolley pole on top of an open deck tram - the pole is insulated from adjacent passengers.

At the top of the trolley pole is a trolley head. Wooden trolley poles are provide with a traction cable running down the centre of the pole.

The collector head is usually the slipper type; this uses a carbon block that sits within a grooved carrier that pivots in both planes. The slipper collector provides quieter operation & less sparking than the older wheel wheel type.

Crucially, the system only works when the head is behind the vehicle as it relies on the vehicle’s motion, which is always trying to return the pole to its neutral axis. Attempts to drive the vehicle in opposite direction will result in the pole leaving the wire.

For this reason the reversing triangle was developed; this uses the natural resistance of the trolley pole to being pushed rather than pulled, to effect an automatic reversal.

You can see a video of that operation at youtube.com/watch?v=d021xWx0Y6…

Crich Tramway Museum Trolley Reversing Triangle

Leeds 399 negotiates the trolley reversing triangle at the Village end of Crich Tramway Museum.

^YouTube

On straight sections of track, support is either by span wires, or (if road width & aesthetic restrictions permit) by using cantilevers. Where these are used, they are typically designed to be aesthetically sympathetic to their surroundings.

At curves & junctions there are more sophisticated span wire connections than on conventional OLE to achieve the required contact wire position; multiple span wires may be anchored at a single point & wire runs may have pull-offs connecting them together.

Special arrangements are needed where wire runs diverge and converge, since the head must be actively guided onto the correct wire.

Unlike conventional OLE, the low tensions and speeds mean that it is practical to split one wire run into two; this is done using an assembly known as a frog, mirroring the terminology used in the track discipline.

Frogs are paired with similar arrangements at track level. Tram systems often used spring-loaded turnouts, held by springs in the dominant direction. Any tram moving through these in the facing direction will follow the normal path.

Trams moving in the trailing direction can approach from the dominant leg, but also from the other direction, the rails moving temporarily into the correct route under the force of the wheel flanges.

For these turnouts a spring-loaded switch frog is provided; this works in exactly the same way, the flanges of the collector head doing the same job as the wheel flanges.

We literally have turnouts in the sky!

Where trams may take either route in the facing direction, active measures are taken to switch the tongue to the correct route before a tram passes. Either the frog is manually switched, using a lever from ground level, or the switch can be automated in tandem with the track.

I've also seen references to trailing frogs with no moving parts - but I don't understand how they would work.

Anyone?