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[Anonymous]

I sort of understand the catenary system. But I don't understand the 3rd rail. Is it like a toy train track with the three rails? And if so,is the the electric pick up the same? Also why are diffrent voltages used,such as 600 volts or 11,000 volts? I would think, there would be a standard voltage.

                                                                                                                dlm

[ndbprr]

Third rail serves the same function as catenary.  It is generally a raised rail on the side of the track elevated about one foot above grade.  It can either be normal side up or down which is called over running or under running.  A shoe usually mounted to the side of the truck has a spring loaded plate that maintains contact.  Subways and elevated lines use this method and virtually any picture will show the thrid rail.  I've never seen it in the center.  Reason being you don;t want someone stepping from the outside rail onto the center rail and becoming a carbon heap.  Most third rails are covered with wood or have a wood guard for this reason.  The higher the voltage the more oomph it has to pass through the wire.  Since most of the European lines are early users of electricity theirs tends to be lower voltage and thicker wire.  Trolley and interurban lines were often built with fairly shaky financing and costly substations to reduce voltage plus shorter runs made using lower voltages a matter of practicality. Some railroads used both.  The NYC used mostly thrid rail but at complex inersections they used catenary since it was simpler to install.  hence the reason for those tiny pantoghraphs on NYC electric engines.

[David_Telesha]

And several New Haven engines had those mini-pantographs in addition to standard pans and third rail shoes...but I don't think it was catenary the NYC installed...as in Grand Central, it was third rail mounted overhead -- not wire.

The 11,000v and 600v differences you mention are night and day. High voltage (11000vAC 25 or 60 cycle, and 25000vAC 60 cycle) will always be catenary and Alternating Current. Lower voltage (660vDC) will be just about always in third rail or light catenary. The reasons are for voltage maintenance and safety (too much current in a third rail isn't very good).

The New Haven was the leading railroad in electrification in the US, between its 1800's trolley's with center track third rail, overhead wires, and best yet, heavy electric catenary from which the PRR copied. The New Haven had many revolutionary locomotives too.. (EP-3 from which the GG1 was based, EP-5 Rectifiers, etc..)

[ndbprr]

It is generally a raised rail on the side of the track elevated about one foot above grade.  It can either be normal side up or down which is called over running or under running.

The original poster asked me in an e-mail to further explain this.  The base of the rail is diffeernt from the head of the rail that the wheel rides on so the wider thinner portion is next to the tie and the thicker head is up for the wheel to ride on.  In third rail it can be either mounted this way or mounted upside down and hung from a wood beam.  In the first case the shoe rides on top of the rail.  In the second it rides on the bottom but the rail is upside down so it rides on the same part of the rail namely the head.

[ndbprr]

Try this URL.  A picture is worth a thousand or more words in this case!

 

http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3DThird%2Brail%2522%26toggle%3D1%26ei%3DUTF-8%26fr%3DFP-tab-img-t-t500%26b%3D161&w=392&h=357&imgurl=www.nia.org%2Fgeneral%2Fimages%2F3rd_r2.jpg&rurl=http%3A%2F%2Fwww.nia.org%2Fgeneral%2Fp_third.htm&size=18.7kB&name=3rd_r2.jpg&p=Third+rail%22&type=jpeg&no=167&tt=8,297&ei=UTF-8

[CSSHEGEWISCH]

Third rail is generally used for lower voltages since a larger conductor is required to get enough energy to the train.  Interurbans got away with overhead wire at low voltages since the demand was generally quite low.

Overhead third rail was used over double slip switches and other long gaps at both Grand Central Terminal and the Detroit River Tunnel electrifications.

[Anonymous]

ndbprr, you are right, the picture is worth many thousands of words and many many thanks to you.

                                                                                                                          dlm

[BR60103]

The Underground system in London, England, uses 4 rails. The running rails are not in the circuit, but the outside rail is 400V from ground and the inside rail is 200V the other side of ground. There is a TV series about the underground that shows new employees being taught how to cross live tracks safely.

[HiWire]

Some of what NDBPRR may have been true in the past.  The new bunch of US light rail systems are usually using wires much bigger than for the same voltages in Europe.  Copper has been cheap in the US, which favored larger spaced big substations.  However if the substations get too far apart, the voltage at the rail relative to ground starts to get risky for people walking around.  Further apart = bigger wires in the air for the same voltage. 

These days for a new system, closer spacing of more substations can almost be paid for by the reduction in cost from using small wires in the Overhead Contact Systems. 

BTW - The generic engineering term now being used in the rail industry is Overhead Contact System, not Catenary System.  The technical term catenary relates to only one of several specific types of  OCS wiring that are used..