Fragmentation of extensive natural ecosystems by roads, railways and other barriers poses major threats to populations of native animals. Attempts have been made to reduce the magnitude of these threats by constructing "underpasses" designed to permit exchange of animals. We compared mammal use of long-established drainage culverts and newly constructed tunnels under the Maldon-Dombarton rail line, near Wollongong, New South Wales. Small mammals used the established culverts, but use of the new tunnels was predominantly by feral predators. We predict that frequent use by small, native mammals will not occur until native vegetation regenerates around the tunnel entrances, establishing a connection between undisturbed vegetation on the two sides of the track. We also argue that follow-up studies such as this one should be an integral part of the environmental impact study of a proposed development.