We've used this a few times, it can be to whichever central London station you're going to initially, for us it's been Charing Cross or London Bridge. The train is quite expensive anyway but not as bad as one adult zoo ticket. Take your tickets to the person on the wide gate to let you through though, in case the automated ticket gates eat your tickets! (Edited to add, In our experience the zoo people will you let you off if one your party forgot and lost their ticket to the gate on the way but you have the return etc!)