by Bay Head Local
Third Rail or Catenary? What are some pros and cons of either/both?
Railroad Forums
Comparison of commuter rail electrification systems?
- General discussion of passenger rail systems not otherwise covered in the specific forums in this category, including high speed rail.
Moderators: mtuandrew, gprimr1
by neroden
There's actually some conflation here. The need for frequent substations is due to using DC power, which is usually used with third rail; this is also the primary reason for the speed limit.
It is possible to use DC power with overhead wiring; this is commonly done in 'light rail' systems where the speed limit isn't an issue but pedestrian safety is.
It is also possible to use AC power with third rail; this is extremely rare and I'm not entirely sure why. The only instances I can think of involve trains which normally run on AC catenary but have to go through low-clearance tunnels; they can be fitted with shoes for third rail pickup in tunnels, and if the third rail is AC they don't need dual-mode locomotives.
Almost certainly the most important feature is system compatibility. If you've got a system, stick with it: using two different systems in one town causes endless trouble later on. (Boston has six, if I counted correctly.)
AC is preferred for most purposes nowadays, because it requires fewer substations, and loses less power during transmission, thus being cheaper over the long run. However, it's substantially more deadly if a person contacts a live wire, so it's often avoided in light rail systems where there's not much keeping people away from the wires except common sense.
Overhead catenary is preferred whenever there are level crossings or a likelihood of trespassers on the tracks, for safety reasons; third rail is preferable on fully grade-separated systems for reasons of durability. (Interestingly it seems that London's Crossrail tunnel will use an "overhead third rail" system which has most of the advantages of both systems; with AC power, because the trains will run off AC catenary outside of the tunnel.)
It is possible to use DC power with overhead wiring; this is commonly done in 'light rail' systems where the speed limit isn't an issue but pedestrian safety is.
It is also possible to use AC power with third rail; this is extremely rare and I'm not entirely sure why. The only instances I can think of involve trains which normally run on AC catenary but have to go through low-clearance tunnels; they can be fitted with shoes for third rail pickup in tunnels, and if the third rail is AC they don't need dual-mode locomotives.
Almost certainly the most important feature is system compatibility. If you've got a system, stick with it: using two different systems in one town causes endless trouble later on. (Boston has six, if I counted correctly.)
AC is preferred for most purposes nowadays, because it requires fewer substations, and loses less power during transmission, thus being cheaper over the long run. However, it's substantially more deadly if a person contacts a live wire, so it's often avoided in light rail systems where there's not much keeping people away from the wires except common sense.
Overhead catenary is preferred whenever there are level crossings or a likelihood of trespassers on the tracks, for safety reasons; third rail is preferable on fully grade-separated systems for reasons of durability. (Interestingly it seems that London's Crossrail tunnel will use an "overhead third rail" system which has most of the advantages of both systems; with AC power, because the trains will run off AC catenary outside of the tunnel.)
by Irish Chieftain
You'll never see any new-build third rail with commuter rail, one can safely assume. Despite advantages relating to access for MOW crews, far lower susceptibility to wind damage than catenary, existing dual-mode capabilities (compared to AC catenary), lower tunnel/bridge clearances et al, the disadvantages such as loss of power during heavy snows, ice storms and floods (both LIRR and Metro-North continue to fall victim whereas NJ Transit keeps running), high number of substations per mile, loss of power in gaps at grade crossings and switches/crossovers (which generally precludes the use of push-pull electric locomotion, but not always since trains running out of Victoria Street in London, England operate thus), safety concerns (at grade crossings and low-platform stations, the third rail is highly accessible), third rail is thus eschewed. Another disadvantage is loss of current through proximity to the ground.
The upper limit for speed with a third-rail system is generally accepted to be 100 mph, this being due to concerns regarding the contact shoe being able to maintain contact with the third rail. Also, AC third-rail is indeed extremely rare; the only systems that currently use it are the metro systems in Moscow, Russia and Pyongyang, North Korea (825 volt system).
Light rail usually uses DC overhead due to concerns with current arc IIRC. Certainly would not be a nice thing to be driving under 25kV 60Hz AC on the street and a high-amperage arc hits your car…
The upper limit for speed with a third-rail system is generally accepted to be 100 mph, this being due to concerns regarding the contact shoe being able to maintain contact with the third rail. Also, AC third-rail is indeed extremely rare; the only systems that currently use it are the metro systems in Moscow, Russia and Pyongyang, North Korea (825 volt system).
Light rail usually uses DC overhead due to concerns with current arc IIRC. Certainly would not be a nice thing to be driving under 25kV 60Hz AC on the street and a high-amperage arc hits your car…
by www123
Irish Chieftain wrote:AC third-rail is indeed extremely rare; the only systems that currently use it are the metro systems in Moscow, Russia and Pyongyang, North Korea (825 volt system).
- All Soviet-type subway systems, Moscow included, have 825V DC third rail.
- Pyongyang has old German U-Bahn cars in service; therefore it is highly doubtful that they have AC third rail there.
by Irish Chieftain
Care to verify your sources? All these sources claim 825V AC for Moscow Metro.
As for the Pyongyang metro, sources are in disagreement over whether the system uses 825V AC or 750V DC—unsurprising for that society (e.g. the lie by the state that the stock prior to the U-Bahn cars was built in NK, when they were actually constructed in China).
As for the Pyongyang metro, sources are in disagreement over whether the system uses 825V AC or 750V DC—unsurprising for that society (e.g. the lie by the state that the stock prior to the U-Bahn cars was built in NK, when they were actually constructed in China).