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How does a Steam Locomotive move?

Our technological journey that progressed us from mechanization, all the way to being globally connected, is one that transcends far back into time.
It's a journey that all began with the humble development of a steam engine.
An engine that was then subsequently developed into the steam locomotive that provided connectivity across the world. 
It is this connectivity that bred the exchange of ideas and resources, eventually sculpting the world to what it was today.

Let’s take a closer look at the key parts of a steam locomotive and see how our mechanical wind up train, The Polaris Express, mimics one very closely.

Mechanical Wind Up Train Model Building Kit

A quick glance: How does a steam locomotive move?

Working principle of a steam locomotive

A locomotive moves by generating enough kinetic energy to power its movements.

It does so firstly by burning a fuel source. At the beginning of the first industrial revolution, the only fuel source was coal - which was eventually replaced by fuel gas.

This burning fuel source transfers the heat to a water containing tank, and heats it up into steam. This steam is channeled into cylinders which are connected to pistons, causing them to move up and down linearly. The steam entering these cylinders are controlled by the use of valve gears.

Finally, as the piston moves, a number of connected rods result in the transfer of linear movement into rotational movement of the wheels.

All of the components thus work in harmony and is crucial for an effective system. If any one part fails in its job, the entire locomotive screeches to a halt.

A locomotive's steam power

Illustration of steam locomotive

If we’re going to get an object to move, we need a source of energy to make that happen.

This source of energy comes from the latent heat of vaporisation of water (produced when water is boiled into steam), and the catalyst: controlled explosions.

Steam engines are an external combustion engine. This means that the fuel does not burn inside the engine.

In fact, the very first practical steam locomotive developed by Richard Trevithick in 1801 used a cylindrical boilerThe boilers work by heating and pressurizing water into steam.

A fireman or stoker places a fuel source such as coal into a firebox. As it burns, it releases hot gases. These hot gases travel into pipes, called flues, placed within the water container and transfer heat to the water. 

Safety valves are built in place to release any excess steam to minimize the chances of a boiler exploding.

Comparatively, in a mechanical wind-up locomotive, the mechanisms involved are very much simplified into a wind up spring mechanism. This spring mechanism allows for storage of elastic potential energy (a.k.a. strain energy as a result of deforming an object) when the spring is compressed. Upon releasing the spring, this stored energy is converted into kinetic energy, resulting in the movement of the wheels.

Today, modern diesel locomotives have internal combustion engines (where the fuel burns inside the engine). They are more efficient than external combustion engines of the same size. This makes them more suitable to be used in vehicles too. An external combustion engine would require a heat exchanger which is too heavy for vehicles.

As such, steam engines have moved on, and are now replaced by a modernised counterpart. However, steam power is still strong, and it is now succeeded in steam turbines, which allows us to generate electricity in power plants.

A locomotive's piston & rods

Steam power engine design

To transfer the energy from the steam pressure into a usable force, we require the use of a piston. Pistons are moving cylinders of metal placed within the engine cylinder, creating an air-tight seal.

A piston moves back and forth along a linear dimension. Steam is forced into the cylinder pushing against the piston and is released when the piston begins moving back.

The piston connected to a piston-rod, and then the connecting rod translates this linear motion into a rotational motion of the wheels. Coupling rods connect multiple driven wheels together.

To ensure energy is not wasted, we need to regulate how much steam enters the cylinder. This mechanism makes use of valves and valve gears that time the opening and closing of the valves mechanically.

Over the years, this working mechanism continues to improve, and now we see them every day in the larger locomotives today.

A locomotive's wheels

Steam locomotive's wheels

Unlike road vehicles, trains do not need to make sharp turns. 

Thus, having a differential in trains, like in vehicles, would be a waste of resources. Instead, the rail wheels themselves are design to act like differentials when negotiating turns, without physically having a differential.

The train wheels are conical in shape, and when it negotiates turns, the whole wheel-set shifts a bit to the right if the track curves left and visa versa. 

What this means is that the axle shifts such that the wheel diameters match the rotation for the speed of the train. A simple yet clever design!

A visual representation of a train's differential can be found here. Richard Feynman, a renowned physicist, also explains this concept eloquently in his video here.

Referencing back to a mechanical wind-up train, you can observe the rail wheels acting like differentials when the wind-up train moves through a bend.

A locomotive's couplers

Railway coupler between train carriages

All locomotives come with couplers. A coupler is a mechanism that connects two objects together. And couplers are essential in locomotives to connect its carriages together.

Safe railway coupling of the locomotives' carriages ensures safety for bioth passengers and cargo.

We see the use of couplers today in our roads when we see trailers connected to vehicles such as trucks.

Couplers connect drive shafts, as well as quick connection couplers for connecting hydraulic devices, cooling devices containing fluids such as fuel, water, oil, and compressed air.

Passenger locomotives often have multiple passenger carriages coupled to the main train engine for pulling.


Building the mechanical wind up train

The steam locomotive behemoths continue to live on in the engineering we see around us. 

From power plants generating electricity due to steam turbines, to the automotive parallels approaching engineering problems in a different way. 

We’ve taken a look at how the locomotive moves and broken it down into the component parts of the locomotive. We also had a look at the history of the industrial revolutions and how steam engines were at the forefront of the first revolution.

Engineering is all about building new solutions to old and new problems.  It is about making things better. And the best way to build that future, is by reliving the past.

Try your hand at building The Polaris Express and experience the wonders of mechanisation for yourself.