What are the engineering principles for a train to get electricity from the railway

Question

How many general methods are there for transferring electricity from the railway to a train? I could see that some trains are connected by a pantograph and some have a third rail.

Are there any other methods? What is the general engineering principle behind each? What are the basic differences (pros and cons) of each method? I was looking for these questions but could not find a good review. If anyone has a reference or could answer them kindly do.

Also, is it possible to build a small model of such a system and scale it up? I am not talking about a commercial model train. I am aiming at self-designing the same engineering principals and demonstrating them on a scalable small model. If so, I would be happy to have a nice tutorial reference.

I am focusing on Electric multiple units (EMU).

Answer 1

Pantograph and third rail are pretty much it.

Engineering principles: both have a conducting surface on the train (moving) in contact with the stationary rail/wire, in both cases you need a material that's resilient and conductive. The contact strip is a wear material.

Differences:

• overhead catenary wires are flexible and will move around when a train drives underneath them. At high speeds, the pantograph can set up waves in the catenary wire, if these waves aren't damped sufficiently, the pantograph will start bouncing.

• third rail is limited to low voltages to reduce the risk of arcing between the rails. The third rail systems I know of (UK), use 750 V DC which limits the amount of power you can draw without excessive losses, so no heavy or fast trains. Overhead wiring can use 25 kV, which has much lower resistive losses. Several power systems are in use for overhead catenary systems, from 1500V DC to 25 kV AC 50 Hz. This patchwork of systems makes it difficult to run international trains in Europe, although with the introduction of high-power electronics it's become easier to build locomotives that can run on 2-4 different systems.

This Wiki page does a decent job of discussing the advantages and disadvantages of third rail systems.

Overhead catenary is more susceptible to damage (from e.g. falling trees, vehicles that ignore the height limit), but has less risk of electrocution on e.g. level crossings. Electrocution risk can be mitigated by interrupting the third rail on crossings, but that has problems of its own (interruptions in the power supply).

Scaling up/down: you can demonstrate the basic principles. In fact, third rail and overhead catenary are both available in standard scale model trains. The pros and cons are more difficult to demonstrate in a model:

• scale model catenary is much less flexible than full size: the wire thickness of the scale model is not in proportion, so the wire is much stiffer.
• scale models use 12V, which has no electrocution risk and arcing is much less of an issue than in the full-size system.
• Due to the much lower speeds and shorter running times, wear in a scale model is much less of an issue than in the full-size system.
• due to the much lower weight, power interruptions are more of an issue in the scale model than in the full-size system.

Answer 2

In terms of safety for the public at large, an overhead cable & pantograph would be used outside in the open so as to prevent anyone, or any forms of wildlife or suburban pets or farm animals from inadvertently being electrocuted by coming into contact with a third rail at ground level.

Where a third rail system is used on the Earth's surface a dedicated rail corridor, either elevated or barricaded, as proposed for maglev systems, needs to be used to prevent issues associated with potential electrocution of people & animals & subsequent issues if the dead or injured entities are on the rails when a train arrives - such as possible derailment of the train or further injury to the entity.

People & animals are unlikely to wander around underground train tunnels thus the chances of being electrocuted by a third rail is less than on the Earth's surface.

Also, underground rail tunnels have limited cross-sectional dimensions - for geotechnical and establishment cost reasons. An overhead electric cable and pantograph would create problems & would need to be very compact. To alleviate such problems it is easier to have a third rail for underground rail systems.

Answer 3

The concepts and various technologies have been known and tried and documented over the last 140 years ...

To give an overview: First, for DC and one-phase AC, you need two conductors (or "connections") between the fixed installations and the locomotive. For three-phase AC, you need three conductors.

The advantages and disadvantages cannot be summed up shortly - there are too many aspects, from purely electrical to maintenance to safety to reliability (weather!) to economic to aesthetic (see "split rail" below) etc.etc.

Here are, off my head, nearly one-and-a-half dozen variants:

1. Siemens' first railway used both tracks for DC supply (one in, one out). One of the disadvantages: If someone gets to stand on both rails, then ...

2. Two slotted overhead tubes(!) - first electric streetcar near Vienna (Mödling to Hinterbrühl) in 1883.

3. A two-rail overhead track, on which one runs a small "car" that is pulled by the locomotive.

4. Centered single overhead wire with contact from below (most widely used today). Disadvantages are of course problems with wind; the thin wire, which is problematic especially with DC systems (see e.g. Italy's system); the propagation speed of the wire's movement (which must be significantly higher than the train's speed - gets problematic with high speed lines like France's TGV); and the reflection of the wire's movement at fixed points (the whole system can be modelled as a wave propagation system, with "mirrors" at fixed points - leading to complex, and in certain cases dangerous, wave phenomena).

5. Off-center single overhead wire; typical for mining railways, where it mst be possible to fill cars from above. Also used on a Swiss line, because the center location is used by a main line overhead line.

6. 3-phase system with two overhead wires, third phase via tracks. Northern Italy had a large "trifase" system for many decades, Matterhorn railway uses it up to now.

7. 3-phase with 3 wires above each other, current collected sideways - used at experimental German railway that first reached more than 200 km/hour in 1903.

8. (Off-center) Third rail at about rail level, with contact on upper side (typical for southern England).

9. Centered third rail (between tracks) with contact from above.

10. Third rail at about rail level, with contact on side (e.g. S-Bahn Hamburg).

11. Third rail at about rail level, with contact from below (new standard for third rails, for safety reasons; e.g. subways in Vienna and Munich).

12. Third and fourth rail (London Tube, for electro-chemical reasons).

13. A third rail below one of the running rails, which is split in its middle for lowering a contact shoe. Was used in Vienna before WW I on the innermost streetcar lines, as the emperor (and others) did not want to see overhead lines purely for aesthetic reasons. There is a video on youtube showing streetcars around 1910 with this system.

14. Overhead third rail - used e.g. in Simplon tunnel (for clearance reasons with truck transport); at Salzburg main station (for aesthetic reasons); on Vienna S-Bahn (I think for maintenance reasons).

15. By a cable on a drum - works only for quite short distances; typical e.g. for cranes.

16. Continuous induction - e.g. Bombardier's "Primove" (coils are only energized when vehicle is above).

17. Induction at some segments or stops, stored in batteries or (newer) capacitors. Used experimentally in some street cars.

Of course, you can "abstract" these various methods to "something touched higher up; something touched somewhat lower; a cable; or induction", if you want. But that would be a layman's idea.

H.M.

Answer 4

The newest generation of trains can be battery-powered, recharging only on or near stations. This is made possible by lithium-ion batteries like those used in electric cars, combined with regeneration when slowing down.

The benefit is that only a small fraction of the track needs to be electrified, saving on infrastructure costs. Stations are located in population centers where electricity is already available.

Answer 5

In scale model trains, the 2 rails used for running the train also power the train.

There are a variety of scales that use this method: O, HO, N, Z, G, and others. Some use a 3rd rail model or overhead lines either as an alternative method or to emulate the trains they are modelling. The brand Lionel typically uses a 3 rail track and is very similar to O scale trains in size.

These trains are often small enough to be run using low DC voltage, where direct skin contact between the rails gives either no, little, or minimal shock value. More advanced, newer trains use Digital Command Control (DCC) systems, this is an AC power system which acts as both the AC power source and has a digital signal embedded in it. The decoder in the train decodes the addressed commands and converts the AC to DC power for locomotive power.

Some larger scale models that can actually carry humans use a small gas, diesel, or oil engine. I'm sure you can use an electric motor, such as something from an electric car, so that's just a minor engineering detail. Getting more voltage and current to the motor through the track might be an issue, though, which is why larger trains don't put electricity though the load bearing wheels. I don't know all the specific details, but putting electricity through a bearing has issues.

I'm answering this way because just as scaling up something like this has drawbacks, so might scaling down another system. Using a 3rd rail or an overhead carriage might not work well for a smaller system, so it'd be easy to consider doing what I've mentioned, only to find out later that scaling it back up doesn't work as intended either. I haven't done research on this, so I don't know if there's a cut-off point or overlap where one system starts to fail and another system suddenly becomes useful. There may also be a speed factor, where a high speed method could be completely different from a low or medium speed method.

My intention here is to make you aware of a system not mentioned by others as well as letting you know that which type of system may depend entirely on what size or speed you want to make your project, so that you will have a good idea as to which system might work best for your intended use.

I'd suggest that you not only ask what systems currently exist, but also ask what worked for people in the size and speed range you are looking at. Asking that 2nd part of the question might end up being another question on SE, seeing as you already have the first part in this question. Linking or otherwise referencing the two questions would be a relevant way to ask that 2nd question.

Answer 6

I didn't see anyone mentioning a keel system. In this system which was used in the past by DC Transit (DC = District of Columbia, nothing to do with voltage), a keel extends from beneath the car through a slot midway between the tracks. A shoe on the bottom of the keel contacts the power rail. This was designed for public safety and the absence of an overhead wire contact. In some respects it must be similar to no. 13 listed above by H.M. Muller.

Answer 7

S-Bahn Berlin and similar use a third rail with alternating mount points, around 60 cm above the rails, they can be mounted that passengers are not in danger - and allow for lower train profile - especially when building such a rail-system in existing city structures. As far as I know the underground trains in Berlin use a similar system