Negative temperatureCertain systems can achieve negative thermodynamic temperature; that is, their temperature can be expressed as a negative quantity on the Kelvin or Rankine scales. This should be distinguished from temperatures expressed as negative numbers on non-thermodynamic Celsius or Fahrenheit scales, which are nevertheless higher than absolute zero. The absolute temperature (Kelvin) scale can be understood loosely as a measure of average kinetic energy. Usually, system temperatures are positive.
TemperatureTemperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. The most common scales are the Celsius scale with the unit symbol °C (formerly called centigrade), the Fahrenheit scale (°F), and the Kelvin scale (K), the latter being used predominantly for scientific purposes.
Sleeping carThe sleeping car or sleeper (often wagon-lit) is a railway passenger car that can accommodate all passengers in beds of one kind or another, for the purpose of sleeping. George Pullman was the American innovator of the sleeper car. The first such cars saw sporadic use on American and English railways in the 1830s; they could be configured for coach seating during the day. Possibly the earliest example of a sleeping car (or bed carriage, as it was then called) was on the London & Birmingham and Grand Junction Railways between London and Lancashire, England.
Railroad carA railroad car, railcar (American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truck, is a vehicle used for the carrying of cargo or passengers on a rail transport network (a railroad/railway). Such cars, when coupled together and hauled by one or more locomotives, form a train. Alternatively, some passenger cars are self-propelled in which case they may be either single railcars or make up multiple units.
Car classificationGovernments and private organizations have developed car classification schemes that are used for various purposes including regulation, description, and categorization of cars. The International Standard ISO 3833-1977 Road vehicles – Types – Terms and definitions also defines terms for classifying cars. The following table summarises the commonly used terms of market segments and legal classifications. Microcar and Kei car Microcars and their Japanese equivalent— kei cars— are the smallest category of automobile.
