车in chinese means bus,car,train,and so on,not the short of 公共汽车。we know it means bus or train or car which like that we know it means车but not 公共汽车 because we understand what the persons who speak to us mean in this dialogue~~~~~~~路车not a word,we say no.3 bus means 3路车 in chinese,no.3=3路,车=bus.so we say 这路车 just means 'this bus'~~~~~~~~~for example u and me here,we see the no.3 bus coming,u ask me 'does this bus go to AA?' i know u mean this no.3 bus,so u don't need to ask'does this no.3 bus go to AA' which like that we know this is 3路车,so u don't need to ask me 这辆3路车到AA吗?u can ask '这路车到AA吗?'