7 aspects of understanding the Metro Rail-Rolling Stock.
Discover the effort that goes into building an engineering marvel.
Metro, the most integrated means of transportation that exists, often goes unappreciated and how it functions is still a mystery for a lot of people. The entire system is developed to relieve the severe stress on the cities and excessive pressure on its transport system. A meaningful and sustainable mass transit system hence becomes necessary to ensure proper transportation facilities are available for the people.
Through this series of articles, I would like to help you develop a sense of understanding of the general functioning and technicalities of a metro rail system.
Being an electrical engineer, I can help you understand the various technical and functional aspects of a metro. The primary electrical engineering components in a metro system include the rolling stock requirement, train operational plan, traction system, signaling & transmission system, substations & power supply, ventilation/air conditioning, and maintenance depot. These are the seven electrical engineering aspects of Metro railways. In MRTS (metro rail transit system), all the above-defined operation is in electromechanical integration. In this article, we will explore the first aspect of metro railways, the Rolling stock.
So let us begin with what exactly is Rolling stock-
General definition- The motorized or trailer carriages, wagon, or rakes operated on a rail track is the rolling stock. The entire metro rail with the main operating body and the wheelset comprises the Rolling stock.
Understanding the difference between a rake and the bogie-
- Rake- Generally, the train itself is termed as a rake.
- Bogie- A bogie is a chassis or framework that carries a wheelset, attached to a vehicle — a modular subassembly of wheels and axles (rod or a spindle attaching the wheelset).
Now let us understand the constituents of the rake-
The rake is what people see and travel in; it’s the train itself. Generally, the rake is of 3 types-
- The driving coach or Driving motor coach (DMC),
- Trailer coach (TC)
- Motor coach (MC).
The three units are attached in a specific sequence to create the rake, the arrangement and the number of coaches are defined based on the population density traversing in a given period of time form one place to another, this in turn also determines the length of the rake.
The recommended and designed sequence of the coaches are-
1. 3 car unit- Considered as the basic unit of the metro system. This system constitutes 2 DMC coaches and one trailer coach. It’s the smallest recommended sequence for metro operation and is preferred in places with low population density.
2. 4-car/ 8 car unit and 6 Car/9 Car train units- Such units are more appropriate for densely populated urban centers of the world. The motorization and coach types are determined based on the various factors of energy efficiency and regeneration level, pantograph, and other electrical equipment’s requirement and motor size required for proper acceleration.
In general, for 6/9 car trains, 66 percent motorization is preferred, which means that out of all the coaches, 66 percent are either driving motor coach (DMC) or Motor coaches (MC) and the rest are trailer coaches (TC). Similarly, for 4/8 car trains, 75 percent of motorization is preferred.
In terms of PHPDT (Peak Hour Peak Traffic Demand value). The metro systems are defined as follows-
Operation of Metro
One crucial fact about the metro is to understand that metros are designed for convenience and not as a faster means of transport. This means that each metro station is designed in such a manner that the distance between consecutive stations is only a few km. Such design enables the widespread use of the metro for transportation.
Hence even though metros are designed to operate at a top speed of around 80–90 km/hr, the average speed of the metro is only about 30 km/hr. This is because as the distance between the metro station is smaller, the metro is usually not able to operate at top operational speed.
Braking system
A conventional braking system is a form of mechanical linkage (brake drum or brake disc, master cylinder or fulcrums, etc.) that turns the vehicle’s kinetic energy into the heat energy, which in turn stops or de accelerate the vehicle.
In the Metro system, we use a set of braking system including-
• An electro-pneumatic (EP) service friction brake
• A fail-safe, pneumatic friction emergency brake
• A spring-applied air-release parking brake
• An electric regenerative service brake
- Provision of the smooth and continuous blending of EP and regenerative braking.
Multiple braking systems are necessary to compensate for situations when one or more braking systems fail and to allow a smooth jerk free experience when the metro is deaccelerating.
The regenerative braking will be the main brake power of the train and will regain the maximum possible energy and pump it back to the system. It’s used in conjugation with Friction-based braking to bring about the train to a standstill position.
Regenerative braking- It is an energy recovery mechanism that slows a vehicle or object by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism, the electric motor uses the vehicle’s momentum to recover energy that would otherwise be lost to the brake discs as heat. This energy is converted to electrical energy by the use of dynamos, and it is then fed back to the power line supplying the power to the coaches, which is generally a 25 Kv ac single-phase supply.
Conclusion-
Metro, in itself, is a very vast aspect of engineering and is integrated into our lives. I can’t emphasize enough on its importance in the development of a nation and its functionality in supporting traffic flow.
From the article, I tried to give you a clear image of what kind of thought process goes through in creating such an engineering marvel, so next time you board a metro, don’t forget to appreciate the effort that goes into developing one. Make sure to check out other parts of the article to get a full understanding of the functioning of a metro.
