Can anyone provide information as to the benefits of supplying power through overhead catenary as opposed to third rail? What are the good and bad points of each? When is one form of power supply more useful than the other. Thanks RC
Third rail is an electric shock hazard to people walking on or across the tracks and can be shorted out by deep snow so it is really more appropriate in subway tunnels or elevated.
Historically, you’d tend to see AC overhead catenary on long-distance mainline electrification because of the ease of transmission and distribution. You can easily step it up and transmit it long distances, then step it down to feed the catenary. The traditional drawback was that you were stuck with AC motors on the locomotives, which are a poorer match for train service than series wound DC motors. You’d see DC third rail in suburban and transit operation where you don’t have great distances involved and the advantage of being able to use series wound DC traction motors for quick acceleration is great.
These distinctions have blurred quite a bit now as there are all kinds of high-power semiconductors for handling and converting AC and DC power.
To expand just a little, Third-rail electrification also limits the voltage of the power supply because the higher the voltage, the greater the distance from any object a spark can jump. Historically the change over from 3rd rail happens above 750V. DC overhead electrifications are fairly common, Netherlands, Belgium, France, Spain, etc. The reason that higher voltage is desirable is due to that fact that power is Watts, and this equation holds Voltage times Amperage equals Watts. To give you an idea of what this means, a SD40-2 is a 3000hp locomotive, 1hp = 746 Watts, so to supply 3000hp to the locomotive you must supply 2.2 million Watts. At 750 Volts that means you must supply nearly 3000 Amps this is requires a very large contact area to avoid melting the contacts (more power than needed to Arc Weld), likely multiple contacts. Now under 25 kilovolt AC electrification, that Amperage shrinks to only 90 Amps. You need better insulation, but much smaller wire or conductor rail will suffice.
Edit to change one reference from Watts to Amps
On MetroNorth and LIRR the speed limit for third rail is 80 mph; has anyone gone faster elsewhere in the world? (With regular trains, I mean-- not tests.)
Nothing significantly faster SNCF had a short stretch of 140kph (87 mph), that was converted, with 20/400 hindsight to 1500V overhead back in the early '70s.
Third rail has to have gaps in it at any grade crossings, and especially at switches/ turnouts. In complicated urban areas, the switches are so close that the 3rd rail can’t be used for longer distances, which presents a risk of a train stopping and getting stranded in a ‘dead zone’ - so a modified 3rd rail of some kind is used overhead instead. That’s why the NYC S-motors had those dinky little pantographs on each end. So you’re essentially using catenary anyway . . .
In some older tunnel installations - esp. the underwater ones - the overhead clearances were too small/ ceiling is too low for safe insulation of the high-voltage catenary. There, side clearances might be enough for a 3rd rail installation instead.
Third rail is typically carried on track ties that just extend out longer to that side by 3 to 4 feet or so, with supporting brackets, insulators, and cover boards, etc. In addition, 3rd rail is a major obstacle/ complication and hence expense increaser for most track maintenance operations, esp. replacing ties, as well as surfacing - not so much for rail replacement, except that really heavy bond wires are needed at any joints to carry all that current (amps). The catenary is up there overhead in the air someplace, and of little concern otherwise except when doing crane work.
- Paul North.
In 1978 or 79 when returning to NYP from Jamaica after a fan trip to Montauk was late into Jamaica, several aboard our delayed connecting train stop watched the mile posts and shouted over 100mph on two consecutive miles. Unofficial. Yes. But…more than one stop watch and…so?
Sounds about right!
So OK, that would have been the LIRR - MU cars ? Which versions/ models, if you can recall ? There was a lengthy article in Trains back then about new equipment and management, etc., and as I remember they were something like M-3’s or M-7’s - the ‘M’ being for Metropolitan - and could have been good for 100 MPH.
| What on earth is this GG1 doing? Trains, October 1965 page 20 Speed test of PATH cars on Long Island ( ELECRIC, LIRR, “MORGAN, DAVID P.”, PATH, SPEED, TRN ) |
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| Parlor car east Trains, May 1968 page 24 parlor cars on the Long Island Rail Road ( “BRYANT, H. STAFFORD, JR.”, LIRR, PARLOR, PASSENGER, TRN ) |
Long Island: Back from Looneyville? |
Actually Paul, it was the brand new M3’s put on for the benifet of us railfans!
Sorry to dump this mess on your post, PDN, but the last ten posts have given me pondering points. I’m playing catch-up. I commuted on NYC 3d rail territory and the idea of going 100 mph + is ludicrous. Methinks the lower (abour 600v DC) doesn’t present much of a safety problem. If it was AC, then “Beware”. LIRR service from Jamaica to Montauk is/was locomotive hauled. No 100 mph third rail there. Wasn’t the Milwaukee Road overhead electrification 1,500 v. DC? BA&P at 2,500 DC? I do wonder what the actual limit for third-rail speed was. As a decendent of James Watt, I thank the poster for capitalizing his surname. I give the same respect to Rudolph Diesel. I’m sure the 100 mph for the M-3/M-7 cars was under 11,000 v. AC. Sorry, Sr. Volta – I should have said “Volts”. I’ll mull over the rest of the posts and be back. Sorry 'bout that! I do have some questions about High Voltage Direct Current (HVDC) transmission. It may be the thing of the future, obviating line-loss. It is used on some of the North Sea oil rigs and generating stations. Might be very interesting…
Hays
It’s harder to pee on the catenary…
but maybe not impossible…?
Hays, we did MU’s…M3’s in fact…from Penn Sta to Jamaica and the diesel hauled train Jamaica to Montauk and back to Jamaica. The M3’s did do at least 100MPH that day by several stop watches and mile posts on several cars. Remember at that time the LIRR did have a test track east of Woodside to just east of Jamaica that allowed for higher than normal speeds. As for regular service, today you can ride either MNRR or LIRR at close to 90 in many places…I have used the sweep hand and mile posts and hav measured up to 89 and 90 on several occasions; motormen and train crews have confirmed they can do up to 90 but no more. And back then, in the late 70’s, the LIRR single track lines were definitely no where near that, often no faster than 65 in some of the few best places. Even the “racetrack” from HIllside to Hicksville couldn’t do better than 60 then. Today I believe the limit is 79 in most places on LIRR and also on MNRR Hudson and Harlem 3rd rail lines, higer on the Shore Line under the catenary.
As another note on the Montauk service, I see in the 1950 era timetables and earlier, that there was not change at Jamaica, that apparently the DD or something hauled it from the Caverns to Jamaica. The Cannonball now starts from LIC or Hunterspoint Ave. via the Main Line to Jamaica.
Heck no, when you get drunken idiots involved anything is possible. One managed to apply for the Darwin Award about 4 years ago doing it off an overbridge.
There were exceptions - Central California Traction had 1200V third rail for several decades, the Michigan Railway tried 2400V third rail but that was unsuccessful. BART is running on 1000V third rail, traditional the third rail was only energized when a train was in the power block.
Hays:
The BA&P was 2400V DC (shared by CN’s Mount Royal electrification). The Milwaukee used 3,000V DC (later upped to 3300V), along with the DL&W, NYC’s CUT electrification and Kennecott Copper’s line between the Bingham pit and the smelter in Magna. The only steam road that used 1500V in the US was/is the IC (along with several interurbans), the SP used 1200V on the East Bay electrification.
Now about current limits:
Third rail was good for up to 14,000 amps (though it may have required several shoes), catenary was good for 4,000 to 5,000 amps (pretty much requiring double catenary as first used on the Milwaukee). With 750V on the third rail, you could get almost as much power as the Milwaukee could get off of their 3,000V catenary, though substation spacing would be a couple of miles with third rail as opposed to 30 miles on the Milwaukee.
When the SP was looking into electrification in the early 70’s, they were planning to use 50kV/60Hz as 25kV wouldn’t support the 30,000hp per train that they were using on Beaumont Hill.
- Erik.
Well, our “dinky little” “S-motors” did have those stubby pantographs. So did the much longer “T-motors”, and the huge (ex-CUT) “P-motors”. Good insurance for not getting stranded on the many ‘puzzle-switches’ of GCT. As far as grade crossings go, I can’t think of a one in the “Electric Division” of the NYC. BTW, the New Haven motors that came into GCT did not have DC pantographs. Never heard of any getting hung-up, but the EP-3s, EP-4s, and EP-5s (“Jets”) were long jobs. Our third-rail ties, about every 6th, or 7th, were only a foot longer than the standard. Methinks third-rail was cheaper, easier to maintain, and safer than overhead high-voltage AC, unless we are talking about hundreds of miles of electric transmission. Remember: don’t pee off the viaduct on catenary! They will have to call out the “wire train” to pick up a ‘crispy critter’. In third-rail territory, the local section crew can scrape you up without disrupting traffic too much, if you happened (rarely) to get zapped.
Hays
Third rail and overhead wire equipment! http://www.sjrail.com/wiki/index.php/Image:P-RSL_MP-2_-6765.JPG
But those stubby pans were only supplimentary for the GCT gaps and not for full main line running. The NH units were road units thus longer than the NYC so would pick up over the gaps. The CUT units had only one pan in the center did they not? Maybe I’m wrong on that. Grade crossings, in your time, I believe you are right. But at least on the Harlem from White Plaines North to Brewster and Southeast (formerly Brewster North) there are several grade crossings.
The Hudson electrification had one grade crossing when I last saw it, near Croton Harmon station. The Harlem has several, north of North White Plains Station. Gaps aren’t a problem because mu trains have only one car without shoes on the gap at a time, and in this case, wiht shoes on both trucks, my memory is the roads are only two lane and thus even one car is never fully gapped. The LIRR has similar grade crossings. I remember the P motors in Cleveland as having the usual two pantographs.
And yes, DD-1’s were used for a wide variety of jobs on the LIRR. As a kid I used to see them on freights, including one into Long Beach. And definitely on the Canon Ball and other Montauk trains with the power changed at Jamaica to the G-5 or K-4 or E-6. Some of the Oyster Bay, Jefferson Point, and Greenpoint trains ran through with change of power also.