Public transport service numbering
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Public transport service numbering is a means of identifying a particular transport service or route.
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Flight numbers
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A flight service can be identified by the airline company, the date and a flight number. This should not be confused with the tail number of the airplane. There are often several flights on a day with the same airplane. On the other hand, the flight of the next day with the same number may be carried out with another plane.
A number of conventions have developed for defining flight numbers, although these vary widely from airline to airline. Eastbound and northbound flights are traditionally assigned even numbers, while westbound and southbound flights have odd numbers. For destinations served by multiple flights per day, numbers tend to increase during the day. Hence, a flight from point A to point B might be flight 101 and the return flight from B to A would be 102, while the next pair of flights on the same route would usually be assigned codes 103 and 104.
Flight numbers of less than three digits are often assigned to long-haul or otherwise premium flights. For example, British Airways flight 1 was the early morning supersonic Concorde service from London to New York. Four-digit numbers in the range 1000-4999 typically represent regional affiliate flights, while numbers larger than 5000 are generally codeshare numbers for flights operated by entirely different airlines.
In the US air crashes are often referred to by their flight number. Elsewhere they are usually referred to by the name of the location of the crash, e.g. Lockerbie. Both in the US and elsewhere, flight numbers may be taken out of use after a crash.
Train numbers
A train service can be identified by the train company, the date and a train number. This should not be confused with the number(s) of the vehicle(s). There are often several train services on a day with the same physical train. On the other hand, the train service of the next day with the same number may be carried out with other vehicles.
A flight number is more often relevant for the passenger than a train number: although the latter are usually in the time-tables, they are often ignored, because most people find it more practical to identify train services by route and departure time.
Bus numbers
Bus services are generally identified by a service number and a destination, although in some areas (notably North America) each service or line is also referred to by a name. In cities like New York City and Washington, DC, there are also a combination of letters and numbers in bus routes. Common letters include X for "express" and A for "abbreviated" (shortened) services.
Metro numbers
Metro services are generally identified by a line number, name, letter, colour or possibly a combination of all inclusive, as well as a destination.