A moving train contains energy, known as kinetic energy, which needs to be removed from the train in order to cause it to stop.  The simplest way of doing this is to convert the energy into heat.  The conversion is usually done by applying a contact material to the rotating wheels or to discs attached to the axles.  The material creates friction and converts the kinetic energy into heat.  The wheels slow down and eventually the train stops.  The material used for braking is normally in the form of a block or pad. The vast majority of the world's trains are equipped with braking systems which use compressed air as the force used to push blocks onto wheels or pads on to discs.  These systems are known as "air brakes" or "pneumatic brakes".  The compressed air is transmitted along the train through a "brake pipe".  Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle.  It can apply the brake, release it or hold it on after a partial application. 
                          An alternative to the air brake, known as the vacuum brake .The vacuum brake is a braking system used on trains. It was invented in 1870 in the USA at the time of air brake, where it enjoyed only a brief period of popularity, primarily on narrow gauge railroads. The system took a greater hold in the United Kingdom, being used there as the primary form of train braking until the 1970s. Vacuum braking is for all practical purposes now a dead technology; it is not in large-scale use anywhere in the world, supplanted in the main by air brakes. Vacuum brakes have now been largely superseded by air brakes which work on a similar principle but use compressed air instead of a vacuum. This allows for more braking power, since the pressure differential between atmospheric pressure and a feasible vacuum is less than that between atmospheric pressure and a realistic brake-pipe pressure.
                                  In a vacuum brake system, depressing the brake pedal opens a valve between the power cylinder, which contains a piston, and the intake manifold to which the power cylinder is connected.  When you apply the brakes, air is exhausted from the cylinder head of the piston. At the same time, atmospheric pressure acts on the rear side of the piston to exert a powerful pull on the rod attached to the piston. When the brake valve is closed, the chamber ahead of the piston is shut off from the intake manifold and is opened to the atmosphere. The pressure is then the same on both sides of the piston; therefore, no pull is exerted upon the pull rod. The brakes are released and the piston returned to its original position in the power cylinder by the brake shoe return springs. HydrovacTM   is a trade name for a one-unit vacuum power-braking system. It combines a hydraulic control valve, a vacuum power cylinder, and a hydraulic slave cylinder into one assembly. This assembly is connected to both the master cylinder and the wheel brakes   and   eliminates   the   need   for   mechanical connections with the  brake  pedal. Pressure on the  brake  pedal  forces  fluid  from  the master cylinder through the check valve to the slave cylinder and to the wheel cylinders. Also, the foot pedal pressure, acting through the master cylinder, acts also against the slave cylinder piston to help the vacuum pistons and pushrods to press against the brake shoes.
                          The vacuum in the brake pipe is created and maintained by a motor-driven exhauster.  The exhauster has two speeds, high speed and low speed.  The high speed is switched in to create a vacuum and thus release the brakes. The slow speed is used to keep the vacuum at the required level to maintain brake release.  It maintains the vacuum against small leaks in the brake pipe.  

The following diagram shows the principal parts of the vacuum brake system as applied to an electric or diesel train. The systems used on steam locomotives were somewhat different.

                The vacuum brake was, for many years, used in place of the air brake as the standard. Because vacuum brake   has the advantage of being simple in design and of having the ability to get a partial release, something the air brake could not do without additional equipment.  The vacuum brake was not as effective as the air brake, it taking longer to apply and requiring large cylinders to provide the same brake effort as the air brake.  It was also slow to release and requires additional equipment to speed up its operation. The vacuum brake can be apply automatic.


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