Trolley Pole v. Pantograph

Sometimes a(n electric) locomotive has two trolley poles up (the front pole swung around to also drag behind) to get a better contact for higher current draw. Same idea with two pantographs up.

FFolz wrote:How are the loads balanced throughout an electric rail network? I imagine it's something of a massive parallel circuit with trolleys as the resistors, but you'd have to have drops in voltage the farther you go from the generator, plus the branch lines have to complicate things.!

Dispatching should take into account the number of trains in each territory served by a substation so that substation is not overloaded.

High voltage lines (sometimes around 10,000 volts AC even back int he 1920's) went from the generator to one substation to another to have a minimal loss (or dissipation) of energy in the lines due to voltage drop. Low voltage (600 or so DC) traction power ran just a few miles out from each substation so each substation territory was something like 5 miles across tops. Each territory was isolated from its neighbors using section breakers in the overhead.

The number of watts needed can be carried as a high voltage and fewer amperes, or a lower voltage and more amperes. If we supply more watts by raising the number of amperes, the number of volts dropped in the wires increases. If instead we raise the number of volts and keep the amperes the same, the same number of volts is dropped. Dropping 50 volts out of 10,000 is a much smaller percentage of energy lost than dropping 50 volts out of 600.

I always wondered what are the characteristics of pantographs that they can handle higher voltages and poles can't?

JLJ061 wrote:I always wondered what are the characteristics of pantographs that they can handle higher voltages and poles can't?

I think the reputation for a trolley pole's not able to handle higher voltages or currents has to do with a wheeled pole where the contact area between wheel and wire is very small and also the wheel bearing is an additional moving contact subject to arcing.

Some pantographs don't have that much contact area between shoe and wire either.

Disney Guy wrote:I think the reputation for a trolley pole's not able to handle higher voltages or currents has to do with a wheeled pole where the contact area between wheel and wire is very small and also the wheel bearing is an additional moving contact subject to arcing.

Some pantographs don't have that much contact area between shoe and wire either.

On a simple design compare the contact area of a wheel cupped around a rod and two rods crossed at a 90 degree angle. If the inner diameter of the wheel matched the outer diameter of the wire you would have a line from one side of the wheel to the other for whatever part of the circumference that the wire touched the wheel. On the pantograph you would have a single point of contact. So it would seem the wheel would have a larger contact area.

But that ignores pantographs with flat contact areas and multiple contact areas. Two flat pieces that make more contact along the length of the wire than a wheel can. With flat contacts you could increase the contact area to as much as you wanted by just making wider (front to back on the train) plates at the top.

One could design a pantograph with less contact area than a wheeled pole but it is not hard to design a pantograph with a much larger contact area.

A typical light rail pantograph has two shoes (bars) about an inch and a half wide each, thus providing a contact area with the wire three inches long.

A typical shoe trolley pole has a flat contact surface about 3 inches long (front to back) but only a thin line at the bottom of the shoe counts because the width of the shoe channel is greater than the wire diameter.

It was some years ago when I last observed but some of the San Francisco MUNI pantographs had a wide section at the middle which I believe was to pass under 90 degree crossing frogs in the trolley pole compatible overhead more readily as opposed to allow a greater current flow.

Pantographs are bi-directional. Trolley poles are not.

Watched Heron Rail's "Singing Wire" vol II a few days ago (DVR'ed off RFD-TV), and they mentioned that the Indiana Railroad (trolley line) came up with a trolley pole shoe that allowed reverse running supposedly without dewiring. FWIW great video but no happy ending.

papabarn wrote:

Pantographs are bi-directional. Trolley poles are not.

Watched Heron Rail's "Singing Wire" vol II a few days ago (DVR'ed off RFD-TV), and they mentioned that the Indiana Railroad (trolley line) came up with a trolley pole shoe that allowed reverse running supposedly without dewiring. FWIW great video but no happy ending.

When wying a car, back-poling is required and expected, and it's not unheard of for historic streetcars to back-pole at 20+ mph for the length of their lines. But, when a trolley pole does dewire in reverse (whether from a kinked wire, a bad shoe or wheel, or ice buildup) it tends to rip down much larger portions of overhead than a pantograph.

Regarding back-poleing: The Crandic (Cedar Rapids and Iowa City) bought an Indiana lightweight, single ended car. The railroad had no way of turning a car at the end, so they developed a unique system.

The pole used a slider in the normal direction. But when running backwards it had a hinge and a little (about 1--2 inches) wheel tilted up. They added a pocket for the retreiver on the side of the car. At the end, there was a wye on the wire only; backing up, the pole would push into the wye and then come out in the trailing direction. THe crew would move the retriever to the pocket and away the car would go.

The device can be seen at the Illinois Railway Museum, where the car went.

In the conduit system the center slot opened into a "vault" below the surface, two steel conductor rails of a special "T" cross section were mounted on insulators below and on either side of the slot (picture these T section rails as two T letters laying on their sides with their tops across from each other). In the conduit system, these T conductor rails were both insulated from ground with the running rails being electrically isolated from the positive and negative T section power rails.

Back poling was quite common at the end of the line on many US streetcar (tram) systems that used single ended cars. Trolley poles equipped with carbon "sliders" can succesfully back through trailing and leading overhead wire "frogs" under certain conditions. As an example, the San Francisco Muni "J" Church St. line used to terminate at Church and 30th sts; Church St. ended in a "T" intersection at 30th St. the southbound cars would right turn onto a straight section of track on 30th St. that crossed church and was only a few hundred feet long and stub ended, the car would then back through the trailing frog along this straight track, crossing 30th the car would back through the leading frog for the northbound track. The car would then be ready for the inbound run after the operators "layover" time.

One property that avoided the use of back poling to maneuver its single ended cars so that they could reverse direction was the Lehigh Valley Transit Co on its 1000 series interurban cars ( former C&LE "Red Devil" high speed cars) used on its Liberty Bell Limited route between Allentown, Pa. and 69th Street Terminal in suburban Philly, and after 1949, Norristown, Pa. The LVT simply installed a second pole on the front of the cars which was used when the car were operated in reverse, either to be wyed at 69th Street, or, after 1949 to turn the cars in Norristown after operation of the line was cut back to that location.. ( The operation in Norristown was interesting in that when a southbound car finished its run at the second story P&W terminal at Norristown, which was single tracked, the car would raise the front pole and operate in reverse a number of blocks through the Norristown Streets to the LVT Freight yard where it could be wyed. It then retraced its route, still in reverse, to the terminal where it entrained passengers for the northbound run to Allentown. Hence the need for the second pole. This operation lasted two years until the LVT shut down completely in 1951.

San Francisco MUNI, specifically, did have poles on the front ends of their single ended PCC's for reverse movements including wye maneuvers.

San Francisco MUNI, specifically, did have poles on the front ends of their single ended PCC's for reverse movements including wye maneuvers.

And El Paso's PCC's had front poles for the long backup move down the street to the car barn.

On unidirectional looped systems, trolley poles have basically have no disadvatages apart from the more critical positioning of overhead wires, the need for frogs at junctions, and the need for insulators even between section served by the same substation. Advantages include simpler construction, and simpler wiring which can be dead straight on straight track.

3rdrail wrote:Trackless trolley poles are made differently. They're a longer pole with a swiveling harp. They've come a long way from the earlier versions, but they still de-wire occasionally, especially at turnouts. There really isn't a way for a pantograph to de-wire unless it snags something that shouldn't be there to begin with. I believe that pantographs started out in popularity in the west on interurbans, but for some reason the rest of the country seems to have gone primarily with poles. Both have their selling points, and one of the pole's is that replacement of a damaged pole can be done in 1/2 hour whereas a pantograph is a project.

Swivelling harp trolley poles would allow greater choice of the positioning of the overhead wires, positioning them to the side would allow shorter bracket arms to be used and even reduce visual obtrusion in some cases. Pantographs can fail too, and I believe that trolley poles rarely dewire on well maintained overhead wires as far as I know.

Disney Guy wrote:Swiveling trolley shoes would allow good trolley pole operation on staggered pantograph overhead, although,
1. Frogs would be needed at junctions with frogs having movable points needed at facing turnouts,
2. The wire would have to not zig and zag as sharply from side to side, for example left, straight, right, straight, left, instead of left, right, left, right, etc.

Switched frogs would probably not only be needed at facing points but also at some trailing points as well, so that triangular junctions could be used for short working (I'm talking about trolley pole use on unidirectional systems), besides I think they would reduce the possibility of dewirements on frogs anyway.

Twin trolleypole current collection (as used on the previous Cincinnati system) would allow the wires to be shared with trolleybuses. Is there any reason why Cincinnati couldn't use it again if their streetcars return?

There were several places in Philadelphia and Toronto, and I assume San Fransisco, where streetcars with single pole current collection and trackless trolleys with 2 pole current collection, shared the same set of wires.