The Best Way to Power Your Railway | Third Rail vs. Overhead Wire

This video a remake of a previous video incorporating viewer feedback and fully revamping the visuals and rewriting the script (for more details, see my recent community post). Please enjoy and consider sharing!

One day there will be a train management sim where you have to decide what type of electrification you use only to curse yourself that you need to adapt your new trains to run on old electrification standards.

It's high voltages that are more efficient at transmitting power over long distances, rather than this being something inherent about AC. In fact, DC is actually more efficient at transmitting power than AC, but this is overshadowed by the difference between voltages - this is why a lot of modern high voltage power links use DC. The reason that high voltages are traditionally associated with AC though is that up until relatively recently, it was just impossible to handle high voltage DC affordably and practically. AC can very easily be changed in voltage using a transformer, whereas DC needs modern power electronics to change the voltage. So why is high voltage more efficient? Well, P=IV tells you that for a given amount of power that needs to be delivered, if your voltage increases, current will decrease (I = P / V). Now if we take R to mean the resistance of the wire rather than the load, P = I^2 R tells us that for a given value of current, power loss in the wire will be greater, in fact proportional to the square of the current. So increasing the voltage dramatically reduces power lost. This is why power transmission lines tend to be very high voltage. OK, so why isn't third rail high voltage if it's more efficient? Simple - because it's close to the ground, the return path, and potentially humans, it's neither safe nor practical to make it much higher voltage than maybe 1000V or so. Most third rail I know of is in about the 500-750V range. So why build third rail in the first place if it's so much less efficient? Well, this comes down to another difficulty of AC vs DC. In the early days of rail electrification, there was no practical way to convert from AC to DC on a train. Why would you want to do this? Again in these early days, AC motors could only be made to go at one speed which was governed by the frequency of the AC power. This is not good for a train, so instead they used DC motors, which are more expensive to maintain but would allow for easy changing of speed by doing things like switching resistors in and out of the circuit. So at this point your only choice is DC distribution - and with DC distribution your only choice at this point was low voltage. The Germans actually came up with a clever way around this - it turns out that low frequency AC can be used to drive traditional DC motors, with a bit of modification. So they used low frequency AC to power their trains, 15kV AC @ 16.7Hz. What's the trade-off here? With lower frequency AC you need much larger transformers to convert the voltage. This trade-off was considered worth it. But in other countries, they went with relatively low voltage DC transmission, either through third rail or overhead lines. Of course, they were often only building very urban electrification systems at first, so the power losses didn't matter much. This is also why they still build new metro systems with third rail - you're just over such a small area that having to have more substations and more power loss is not worth the hassle of going with AC. But for long-distance lines, the power losses start to get significant. Then around the 50s, technology like mercury arc rectifiers, soon after giving way to solid state rectifiers of various sorts, could now be fitted on trains. This would allow AC to DC conversion to happen on trains. This means you can now have high voltage efficient transmission at normal mains frequencies, convert it down to low voltage for your motors, and rectify to DC. This is about when the 25kV AC @ 50Hz (or 60Hz depending on the local mains frequency) standard emerged for overhead wires, a standard used across large parts of the world to this day. Of course many countries still retain their legacy systems as well, and perhaps only use 25kV on new high speed lines, if at all. But it's heavily used in large parts of France and Britain, and many places that didn't have a serious electrification programme until a significant time after WWII. As far as I know after the technology was proven in Britain and France, no country has chosen anything else for a completely new-build electrification system for a long-distance rail network where there hasn't been a need for compatibility with some existing electrification. What else has changed? Remember I said that DC motors are harder to maintain than AC ones? Well, advances in power electronics in the 1990s have allowed DC motors in trains to give way to AC motors. This basically involves using power electronics to change the frequency of the AC as required to change the speed of the motors. Nowadays most trains, even ones designed to run on DC power, will have AC motors; obviously the DC will be converted via power electronics to AC to power the motors. Many trains since the early 90s and most trains since the 2000s have been built like this.

One of the main benefits of electrification (vs systems where the power is generated on board the train), regardless of the mode of collection. is the ability to adapt the power supply to different sources of generation without having to change your fleet of rolling stock. You can switch to cleaner, more efficient technologies as they come along, and the railways and transit providers are unaffected. You can contrast this with the costs North American freight railways have had to bear as new pollution standards for diesels have been implemented. New standards mean newer, more expensive locomotives.

If anyone else is curious like I was, the reason why the pantographs don't ruin the wires and themselves with friction when trains are going at such high speeds is the contact point uses graphite. Graphite is both a solid lubricant and electrical conductor. When it wears down they simply replace the contact point on the pantograph and the wires above are just fine.

This may be a bit of a controversial view, but I actually like how overhead wires look, along with the pylons. If done right, it can give a sort of futuristic feel to a railway.

I've seen the MBTA blue line change modes from third rail to catenary! It happens at airport station

I work for the Washington DC Metro. I just wanted to point out a few details about 3rd rail you missed. 1. The main reasons 3rd rail switches sides is to separate sections so they can be shut off in individual segments instead of the whole line, also it evens out the wear on collector shoe. 2. On a bottom contact system the top and sides are still live as it's a solid piece of metal, also the bottom and sides of the third rail are the most likely places to accidentally touch with your foot, not the top. 3. For inclement weather there is heater tape on the rails. Also we have special deicer flatcars that have insulated scrapers to scrape ice off the 3rd rail and spray nozzles for glycol antifreeze. Additionally in extreme conditions the revenue cars can also be outfitted with glycol sprayers. 4. In the yards there is 3rd rail. However inside the railcar shops there is something called a "stinger", which is basically a 750 volt jumper cables hanging from a track on the ceiling. It's got a clamp on the end that a mechanic wearing insulated gloves will attach to a collector shoe. This is probably what they're talking about for the Chinese metro yards.

Interesting. I like the idea of using third rail below ground and then swapping aboveground to make it safer for people. Also the idea of having the power lines oscillate to spread wear out. That's one of those subtle little design features that I'm sure was a big "oh duh" moment when someone came up with it.

Interesting fact: only third rail subway lines have their well known smell because the graphite from the contact shoes wear out and the poor ventilation on sublevel lines create the distinctive smell

10:40 this approach of using catenary in the train yard to protect the workers is used on the Bucharest Metro system in Romania. On normal service, the Bucharest Metro uses third rail power at 750V DC, while on train yard it is used low-power overhead lines at 230V DC, where the maximum speed is at 15 kmh!

Rigid Catenary in urban tunnels with regular catenary in the open seems like the best overall option to me. Does anyone know what Crossrail uses in tunnels? It seems to be rigid catenary but with catenary poles? Odd

It's basically the battle between the 'coat-hanger and knitting' or the 'stabiliser rail'. To be honest, nowadays it's easy enough to have trains that can do both. A lot of UK trains either have both fitted, or are designed so that the other system can be retrofitted relatively easily.

It may just be me but I like the appearance of the catenary. In Germany the Green Catenary poles have become a Staple and it's hard to find anyone who actually considers them obtrusive

Technically Catenary refers only to the top of the 2/3 wires in a typical OHLE system due to the shape it follows. The wire that the pantograph actually touches is called the contact wire (and droppers connect the 2) When there is only 1 wire as in a tram system its generally referred to as a trolly wire regardless of whether pantographs or trolly poles are used (or both as per Toronto)

Watching this as a Chicagoan I didn't realize how unusual it is that Chicago has an entirely uncovered top contact third rail for the entirety of the 'L' , which even includes a lot of grade crossings on the Brown, Purple, Pink and Yellow lines.

This electric rail system has been interesting me since my childhood, and your video has given me the essential information to understand its work procedures

Pre-60s trams extensively used centre-aligned 3rd rail in many cities around the world, fun fact

You missed one of the best examples of a train system that uses both overhead wiring and third rail on the same service: The New Haven Line of Metro North Railroad, which serves New York City and its Northern suburbs including into Connecticut. Running from Grand Central Terminal in New York City, the New Haven Line uses third rail power at 700V DC to just outside NYC at Pelham, NY, where it switches to overhead line at 12.5kV AC. Just a few miles further at New Rochelle, NY, the line becomes shared with Amtrak long distance services joining it from a line from Penn Station to the South, which also use AC overhead electricity - Metro North and Amtrak services share the rest of the line from New Rochelle NY to New Haven, Connecticut using said overhead line all the way.

Thanks RM! Love all of your videos but especially love these educational breakdowns. As a transportation geek in training I found this extremely helpful 🙏🙏🙏