Vacuum brake

Vacuum brakes also enjoyed a brief period of adoption in the United States, primarily on narrow-gauge railroads.

Their limitations caused them to be progressively superseded by compressed air systems starting in the United Kingdom from the 1970s onward.

The automatic vacuum brake was slightly more expensive to manufacture and install than the simple system due to it requiring a higher number of machined parts, and incurred higher running costs since the ejector ran continuously (at a cost in steam and thus fuel and water) to maintain the vacuum when the train was running, rather than only being used when braking as in the simple system.

In this accident at Armagh, a portion of a train was detached from the locomotive on a steep gradient and ran away, killing 80 people.

The cylinder rocks slightly in operation to maintain alignment with the brake rigging cranks, so it is supported in trunnion bearings, and the vacuum pipe connection to it is flexible.

When a locomotive is coupled to the vehicles, the driver moves the brake control to the "release" position and air is exhausted from the train pipe, creating a partial vacuum.

Air in the upper part of the brake cylinders is also exhausted from the train pipe, through a non-return ball valve.

The GWR favoured this due to the use of braking systems working on a vacuum level higher than other railways (see below) which would have required a relatively large and steam-hungry "small" ejector.

This allowed the driver of the lead engine direct control over the brakes on any trailing locomotive (as well as the train itself) when double heading.

BR's Modernisation Plan of 1955 called for, amongst other things, a long-term aim to switch to air brakes for both passenger and freight stock.

Sea level air pressure is about 30 inHg (762.0 Torr; 101.6 kPa; 14.7 psi), depending on atmospheric conditions.

On steam engines this was usually a reciprocating steam-driven compressor, which was quite bulky and much more complicated and maintenance-intensive than the vacuum ejector, which was compact and had no moving parts.

Another disadvantage of the earlier air brake systems (although later overcome) was that it was impossible to make a partial release.

A corollary of this was that the standard vacuum brake system, as used between the 1860s and the 1940s, could not easily maintain a constant level of application.

The only way to do so was to carefully balance the setting of the brake valve and the small ejector, which was difficult to achieve in practice and was not even possible on some systems which combined both into a single control.

This meant that braking occurred through a series of controlled applications and releases – perfectly adequate to safely bring a train to a halt but requiring constant management to maintain a speed on a downhill gradient.

The Big Four companies formed in 1923 all chose to adopt the vacuum brake as the new standard for most rolling stock, with the same 21 InHg operating vacuum with the continuing exception of the GWR (and many electric multiple units introduced in this period used variations on the automatic air brake).

However, air-braked steam-hauled stock remained in service on the former Great Eastern Railway suburban lines from London Liverpool Street until the end of steam on the GE in 1962.

A train of stock due for maintenance could make it difficult to maintain a vacuum, even requiring intermittent use of the large ejector when running.

Entry and exit ports at the top and bottom of the cylinder, where the rotor is furthest from and nearest to the wall, provide a vacuum pumping effect.

They are cheap devices, extra pumping capacity can help to release the brakes more quickly and their redundancy reduces the risk of a failure causing a failed train.

On a smaller vehicle such as a traditional four-wheeled goods wagon, it is much easier to fit just one kind of brake with a pipe for continuity of the other.

Train crew need to take note that the wrong-fitted wagons do not contribute to the braking effort and make allowances on down grades to suit.

Many of the earlier classes of diesel locomotive used on British Railways (and electric locos up to and including the Class 86) were fitted with dual systems to enable full usage of BR's rolling stock inherited from the private companies which had different systems depending on which company the stock originated from.

If these taps are incorrectly closed, a loss of brake force may occur, leading to a dangerous runaway.

[8] Twin pipe vacuum systems were standard on the first generation British Rail Diesel Multiple Units which replaced steam locomotive hauled passenger trains on many branch and secondary lines in the 1960s.

The second "high Vacuum " pipe and associated reservoirs and valves were used as a means to increase the speed of the release of the brake.

South African Railways (Spoornet) operates more than 1 000 electric multiple unit cars, which are fitted with air compressed brakes.

The application and release valves greatly increase the rate of train pipe vacuum destruction and creation.

Iarnród Éireann (the national rail operator in the Republic of Ireland) ran vacuum-braked British Railways Mark 2 stock on passenger trains until the end of March 2008[10] and still operates vacuum-braked revenue freight (at least in the case of Tara Mines ore traffic).