As the title states, in the rail industry, why are the majority of railway's third line or overhead line in DC volts and not AC volts? My initial guess would be that it is in AC since it is easier to distribute AC over long distances. I do know there are a few overhead lines that are 25KV AC, but the majority are 600-750 V DC.
EDIT: I found this article which explains the difference a little bit, but it still doesn't explain why the majority are DC.
Railway infrastructure is expensive. It is relatively rare to create totally new tracks and when you do, they most often conform to the engineering norms of the existing tracks (gauge etc) to allow for flexibility in rolling stock usage etc.
Therefore decisions about electrification were made in the 19th century (e.g. 1890 in London). At that time, speed control of large motors was probably easier for DC than for AC where the speed is linked to AC frequency.
Also at that time DC distribution had advantages over AC.
Subsequent technological revolutions are generally hampered by the need to maximise return on very long term investment in large-scale infrastructure.
An interesting case is London's Thameslink which has trains that operate on overhead 25KV AC for the northern part of the journey and on third-rail 750V DC tracks for the portion of the journey south of Farringdon station. The costs of introducing incompatible infrastructure can be considerable.
This is actually a very interesting power electronics question; none of the answers have hit all the major points:
Regardless, we need DC to drive motors
Therefore, the question is: Where do you put the rectifier?
Rectify AC to DC on the train and use HVAC at 25 kV to get the power to the trains
Pros:
Cons:
Rectify track-side and use 600V-3kV DC to transmit to trains
Pros:
Cons:
I recall reading about a Russian experiment in the 80s that compared solutions 1 and 2 above and found that despite the losses in the transmission line that the overall system was more efficient with DC transmission due to the power electronics required. Nevertheless, many regional trains and high speed rail do use HVAC.
There are some other considerations:
Note that power is not usually transmitted long distance along the track (especially for medium voltage DC): the lines are fed along the way, not just from one end.
The whole of the south eastern end of the UK use the third rail system - it was never used anywhere else in the UK and I believe a major reason was that a lot of this area is urban with low bridges hence a third rail system. DC overhead lines (5kV) were used along an old stretch from Manchester to Sheffield.
DC control is one aspect but there is another and that is induction to track control and telephony systems. An AC third rail would represent a big source of magnetic interference for track signalling and track telephone systems. Originally the signalling and track control was done mechanically so AC wouldn't be a threat so this "reason" is more a 20th century explanation rather than a 19th century one.
However, track-side telephone systems would have been affected by AC from the onset and, because the voltage is lower than overhead AC power feeds, the current would be higher and induction greater. A third rail is much closer to the track-side telephone wires as well making things worse.
As an example, when the UK's east coast mainline was electrified (overhead), engineers reported that telephony problems were occurring on lengths of cable about 1500m to 1700m (1 mile) or greater. For a third rail where the current is probably going to be at least ten times higher than overhead 25kV systems and about one-third the distance from cables you can guess AC just wouldn't work even on short distances.
HVAC (High Voltage AC) has some disadvantages, like distance, for instance. Although the transmission of AC is easier and widely used to transfer power from home to home, it is not used when there is one long transmission line.
HVDC is better here because there is almost a constant efficiency of transmission along an HVDC cable. The break even point between HVAC and HVDC is around 50km. If you get greater than 50km it will be more beneficial to use HVDC because of price and efficiency.
Here is a graph between HVDC and HVAC comparing price and distance.
Since these trains are going long distances it is better to use HVDC.
Another real life example would be offshore wind farms. Because they are so far offshore they use HVDC and transmit more power on land than they would with HVAC.
Here's an article on offshore wind farms.
Bit late to the party, but a reason to stay with DC (or build new) is regenerative braking. Most electric multiple-unit trains today will reverse the motors to slow down, dumping the generated power back into the supply line. Very easy with DC as all you need to regulate is the voltage. Syncronization nightmare with AC. If the service is busy enough theres always something else running in the same electrical division.
If we widen our horizont over to the non english speaking area we could conclude this - there seems a multitue of reasons:
In Europe, most all Trains do have overhead lines and they range 3kV, 15kV and 25kV AC - Depends a bit when and who build it. (not counting some alpine local trains which even exist 3 phase!) . On the other hand most if not all Trams in cities do use DC (around 600V).
One reason for higher voltages are more energy transport. This is true for railway trains, as they use up to Mega Wats when starting or sprinting. Thus 25kV will need less Amperes and thus less metal - an over head line could be used instead of a third rail. But why then AC. Because at that time only AC could handle higher voltages due to the existence of transformators. DC gave big problems.
Looking at a third rail, what distance to the ground would you need to carry 25kV in it. Quickly it seems not very practical. Therefore rather use 500-750V only.
But still many trams and subway trains in the cities (Zürich, Berlin, München, Wien, ..) do use 600V. So why is this? This goes back to the very early days, when only DC motors were available/thinkable. (Actually speed regulation was done by big switchable resistor arrays on the roofs). Still you needed more than 110V on the line as there was in the grid to power that bigger coach. So they decided for a bigger voltage extra produced for that purpose. Later Mercury steam rectifiers were used. But 600V is still handle able. But you need big switches with wider distance to kill the switch off arc. This is also the reason why not to use DC with more than 1kV easily.
Looking at the origins of the train system in DE,CH,AT,SE and Norway which is 15kV 16,7Hz (fun stuff): this has it's origin from a train line built between Innsbruck (AT) and Garmisch Patenkirchen (DE). They chose electricity because only so they could climb the steep mountains up. At the time technology had few choices. The best motor of choice with the best efficiency was 16,7 Hz and about 15kV were good for transmission of the necessary wattage. That has then persisted until now. In fact in the mentioned countries there is a whole extra network for train power transmission. It uses 110kV or 65kV (CH) for wide range transport.
That may give you an impression about the reasons why stuff was chosen back then.
