This paper describes an experimental and numerical study of an intrusion propagating along the interface of a two-layer fluid in a channel. We find that when the density of the intrusion is the depth-weighted mean of the layer densities the interface ahead of the intrusion is undisturbed, but for other densities the interface ahead of the intrusion is displaced vertically. We find that this vertical displacement, which takes the form of an upstream-propagating long wave, depends on the properties of the intrusion and not only on the relative depths of the two layers. For the case when the interface is undisturbed the intrusion propagation speed is a minimum. We develop an energy argument that describes the observed variation of the intrusion speed from this minimum speed as a function of the intrusion and layer densities and the ratio of the layer depths. We also show that if, and only if, the layer depths are equal, the speed of the intrusion is independent of the density of the intrusion.