• Data reference period
• Geographic localization (Portugal)
• Type of trains
• Rail segments

• TRAIN:  One or more railway vehicles hauled by one or more locomotives or railcars, or by one railcar alone, running under a given number or specific designation from an initial fixed point to a terminal fixed point.
• ECONOMIC TERRITORY:  Territory that consists on: (a) the geographic territory administered by a government within which persons, goods, services and capital move freely; (b) any free zones, including bonded warehouses and factories under customs control; (c) the national air-space, territorial waters and the continental shelf lying in international waters, over which the country enjoys exclusive rights; (d) territorial enclaves (i.e. geographic territories situated in the rest of the world and used, under international treaties or agreements between States, by general government agencies of the country (embassies, consulates, military bases, scientific bases, etc.); (e) deposits of oil, natural gas, etc., in international waters outside the continental shelf of the country, worked by units resident in the territory as defined in the preceding subparagraphs.
• TOTAL DISTANCE TRAVELLED:  The total distance travelled, laden and unladen, by a vehicle with the exception of the distance travelled by a road goodsvehicle when carried by another means of transport.
• REFERENCE PERIOD:  Period to which the information refers and which may be a specific day or a time interval (month, fiscal year, calendar year, among others).
• RAILWAY NETWORK:  All the railways in a particular area.

(Average distance travelled by train = no trains-km/no trains)