Question: In the sentence below, what does "one" refer to in "one pushing the other"? Does it refer to the girls or to the cars?

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> On the way into town one day I noticed a couple of young girls working their way along the lines of cars waiting at a busy junction, one pushing the other.
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Answer: 
In English, the phrase *"one pushing the other"* can only refer to two things, the *"one"* and *"the other"*. If there were more than two things, the phrasing should have been *"the others"* or *"pushing one another"*. Thus, it cannot refer to the lines of cars, which clearly contain a lot more than two cars. 

It must be that one of the girls was pushing the other, although it's not entirely clear how (maybe in a wheelchair; maybe not).
> 
> on the way into town one day I noticed a couple of young girls working their way along the lines of cars waiting at a busy junction, one pushing the other.
> 
> 
> 

One girl is pushing the other girl. As we know nothing else about the girls, "one" just means one of the girls -- the one pushing.