The walkthrough can only be accessed by riding the road train (which is also the only reliable way of seeing the Black-backed Jackals). You are taken through the exhibit by the train driver, and can either then ride back to the station or exit the lemur enclosure through a second exit, emerging near the lions. To be honest, if I ride the train again next time it'll be for the jackals - the lemurs can be viewed perfectly well from all the old white tiger viewing areas and the whole procedure is a bit of a faff.
