maa(pinna) all, maa sees asuv või toimuv

an electric railway for the transport of passengers with the capacity for a heavy volume of traffic and characterised by exclusive rights-of-way, multi-car trains, high speed and rapid acceleration, sophisticated signalling as well as the absence of level crossings to allow a high frequency of trains and high platform load. Metros are also characterised by closely spaced stations, normally meaning a distance of 700 to 1200 m between the stations. 'High speed' refers to the comparison with trams and light rail, and means here approximately 30 to 40 km/h on shorter distances, 40 to 70 km/h on longer distances., inimeste veoks ehit. tänavaväline linnaraudtee. M-l on suur veovõime, liiklus on regulaarne ja väga kiire. M-liinid kulgevad enamasti maa all (tunnelites), harvemini maapinna kohal (estakaadidel) või piirestatult maapinnal.