You get 2 trolleys with 2 coins. Then as you connect one into the other, the coin comes out, which you use for the next trolley, and so on. Once you've added the last trolley, the coin comes out, then you connect the last trolley to the first one, and the other coin comes out.

If you want to short cut the tedious 'getting one trolley at a time' thing, use one coin on the first trolley (the one that's chained to the park itself) in the trolley park and take the whole string of them in one go.