3rd Rail or Overhead Catenary?

[Ochanomizu]

Hello, 

 

A colleague has posed a question I could not answer.  He accepts that 3rd rail might save space in tunnels.  He accepts that overhead catenary might be safer when it comes to personal safety and preventing accidental electrocution.  However, what other factors influence the choice of system to employ on a railway?

 

I could not answer this question.  I am also curious to know.  I had thought 3rd rail was simply a space saver for tunnels with low height, but my colleague pointed out many instances where 3rd rail is used above ground.  Why?

 

I  then proposed that maybe 3rd rail used less material.  I believe the actual overhead wires are not copper, but some form of aluminium alloy.  Perhaps there is a saving there, but it terms of actual steel, there might be more in the third rail than in the posts that support the catenary, especially on straight sections of track.

 

It seems to me, and I'm not in the industry, that issues of labour, weather, and maintenance are all similar and cost about the same.

 

One marked difference, and the reason my colleague posed the question, is aesthetics.  You must agree that 3rd rail looks neater.  If there is no overarching reason in favour of catenary then I have a question of my own: Why not rip the whole lot out and replace it with 3rd rail, which would look much better?

 

 

[katoftw]

Third rail is DC only and to 1200V max.  Old school power supply stuff.  Something to do with the resistance of the large bulk of the 3rd rail.  Overhead wires have less resistance and allow higher voltages and AC.

[railsquid]

Also: lower maximum speed.

[kvp]

3rd rail is more effective at low voltages and higher currents and allows cheaper rolling stock but only allows low voltages.

 

Catenary is more complex but allows higher voltages and speeds thanks to being tensioned. Also it's continous, while 3rd rail has to be interrupted at every turnout and crossing.

 

There is a 3rd option though, overhead 3rd rail that is both space saving, simple, cheap and safer than side rails. This was used on the first fully electrified subway line in the world (second subway including steam lines). The line is still in use with the same technology today in Budapest. The voltage was the industrial standard 380V DC which has been raised several times since the 19th century. Imho some overhead DC lines also use overhead 3rd rail in tunnels even in Japan.

 

Overall i think side running 3rd rail is more of a legacy technology or used mostly where catenary is not desired for aesthetical or cost reasons.

[Claude_Dreyfus]

Most of Southern England is electrified using 3rd rail; 750v DC. The origins of this goes back to the 1900s when electrification started south of the Thames to compete with the tram network. The two main protagonists used different solutions. The LBSCR (London, Brighton and South Coast) ironically used AC overhead for the lines out of Victoria towards Croydon and for the South London line. This was because they envisaged electrifying their main line to Brighton.

 

The other company, the London and South Western, adopted third rail. Cheaper and quicker to lay. By 1923 the two companies had been formed into the Southern Railway. Senior managers were from the South Western, and by the late 20s the Brighton overhead wires were gone.

 

There has long been talk of converting the third rail lines to overhead, but I am not convinced. The UK cannot manage electrification well any more (see the continued issues with the Great Western main line project). Third rail is quite reliable, but being at track level is more susceptible to snow, flooding and ice.

[railsquid]

Interesting, I never knew that about the LBSCR overheads.

[bikkuri bahn]

Another good thing about overhead catenary is that it allows (or at least makes it much easier for) through operations between metro and suburban rail, like the S-bahn systems in Germany and Tokyo Metro/Toei and private railways. 

[trainsforever8]

Another good thing about overhead catenary is that it allows (or at least makes it much easier for) through operations between metro and suburban rail, like the S-bahn systems in Germany and Tokyo Metro/Toei and private railways. 

Really? I didn't know that Germany had an S-Bahn line doing through operations with a metro! Which one is it?

[railsquid]

I wonder if the Karlsrühe Stadtbahn is what's meant here? I'm not aware of any classic U-Bahn/heavy rail S-Bahn interoperation.

[bikkuri bahn]

I meant "metro" in the geographic sense of running underground through the city center with stops in major neighborhoods, not the technical sense of self-contained metro characterized by short station spacing and incompatible with mainline running.

[kvp]

Good examples are the transilien/rer/ratp lines in Paris. It's an s-bahn system that runs like a metro under the downtown and uses various ac/dc overhead systems so trains may switch systems several times during a trip.

 

A short part of the Budapest suburban network is also overhead in tunnel on the inner end of the Szentendre line with diesel trains from the interconnected Esztergom line terminating at the last surface station. Most outer stations are low platform with no crossing gates across the tracks, just a sign to watch for trains before crossing. Line speed is only 60 km/h around these unsecured crossings.

[Melandir]

In Milan there are two line currently using the third rail, on is the red line (the oldest line) and the other is the new line 5 that is fully automated

[HantuBlauLOL]

flooding will kill 3rd rail trains.

[Suica]

flooding will kill 3rd rail trains.

Well... Flooding will kill just about any modern rail system.