Transport infrastructure investment is an important element in the creation of an efficient and sustainable transport sector. Intermodal flows are seen as critical to rail freight’s future success and feature strongly in contemporary transport policy. This paper investigates a practical rail network enhancement scheme from the United Kingdom (UK) designed to achieve a shift of containers from road to rail. The paper’s aim is to determine the effects on rail freight efficiency of a loading gauge increase in April 2011 which allowed 9′ 6″ high containers to be transported on standard wagons on the corridor from the port of Southampton to the West Midlands. The study is based on a “before and after” survey of container train capacity provision and load factors, with the “before” survey taking place in 2007 and the “after” survey in 2012. A consistent approach to data collection in both surveys allows detailed analysis of original survey data to be carried out. It is clear that there have been considerable improvements in both on-train capacity and train loads between 2007 and 2012, and that these improvements have been greatest on routes that have benefited from the gauge enhancement. Rail’s mode share of container throughput at Southampton has increased. There have also been wider benefits to off-corridor locations served from Southampton and elsewhere across the network. Overall, the impacts on rail freight efficiency of the gauge enhancement have been substantial, with efficiency improvements evident even at a time of economic stagnation.