why do subway trains need only one electrified third rail? I thought electricity needs a negative and positive? shouldn’t we need two electrified rails?


why do subway trains need only one electrified third rail? I thought electricity needs a negative and positive? shouldn’t we need two electrified rails?

In: 2

The other two rails are grounded. That forms a circuit, but the rails themselves aren’t powered/dangerous.

The other two rails can act as the negative or neutral connection. That way you don’t need a 4th rail. Also since axels are usually connected you cannot just use 2 rails as a positive and a negative.

Some systems do – the London Underground for example has 2 electrified rails.

Generally though negative current is handled through the 2 running rails. A complexity is current leakage from these which can be problematic in built up areas ( why they adopted 4 rails for London in the 1880s) which is now handled by sophisticated insulation between the running rails and the support.

DC has two wires for negative and positive electricity.

AC has two wires – one has the zap, the other one goes to ‘ground’ or neutral.


EDIT: Depending on style of train, they either run on DC or AC, although i believe all trains that dont use diesel generators and get electricity from the rails are AC.

It’s been a long time since I studied this but…

Think of electricity as a water fall.

The height of the water fall is the voltage, the amount of water coming over the water fall is the current.

If the water fall is 1km high but only a trickle if water is coming over the waterfall then you are not going to die.

If the water fall is just a centimetre tall but a torrent is coming over the fall then you are going to be washed away and die.

So high voltage isn’t an issue, but high current is.

Voltage is also describe as potential difference, the difference in height from the to top of the water fall to the bottom.

If there is no difference in height then you don’t have a waterfall, you have pond, no current can flow and therefore it’s safe.

So the ground neutral rails are at the same voltage as you, so no current can’t flow so it’s safe to touch the neutral rail.

Let’s now talk AC vs DC.

Firstly there is no such thing as negative and positive electricity, there is only electricity.

The confusion here is a negative and positive voltage.

To understand negative voltage imagine you are standing next to a river.

The river and you are at ground level.
We somewhat arbitrarily refer to this a zero volts.
Rather than think of this as zero volts it helps to think of this as a zero potential difference (pd).
The river could be at the top of a mountain but so are you so the pd is zero and so the voltage is zero.

If you walk up the mountain, away from the river, as you walk up the mountain the pd between you and the river increases.
If you walk down the mountain the pd decreases.
When you go up the mountain we can call this a positive pd, so we would refer to going down the mountain as a negative pd.

So now we have negative and positive voltage.

For DC the active rail is held at a consistent positive vintage to the neutral rail.

For AC the voltage is continuously changed from a positive voltage to a negative voltage. This change typically happens sixty times a second.

It doesn’t matter if the voltage is negative or positive, only that it is different to you.

If there is a difference it will allow current to flow and if there is sufficient current you will die.

Train lines have a LOT of current.

So for both AC and DC rails you have zero pd between you and the grounded rail a non zero pd between you and the active rail.

One last note.

Electricians take about leakage.
This is where the grounded rail is not properly earthed, for instance this can happen if the rails connection to the ground becomes rusty.
When this happens the grounded rail can become an active rail, so in the end it’s better to touch neither rail as on a bad day any of them can kill you.