Built between 2010 and 2011 by Bombardier at their Derby factory, the Class 172 DMUs have had success on diesel operated inner-suburban services in and around London and Birmingham.
A total of thirty-nine units were built, eight for London Overground, and four for Chiltern Railways. These twelve units have no inter-unit gangways, and two carriages. The remaining twenty-seven were built for London Midland, these have inter-unit gangways, which gives the units an electrostar-style look.
The units are somewhat unique on the modern railway, in that they have very distinctive sounding mechanical transmission, which is noticeable when the train accelerates and decelerates, as the gears can be heard shifting, just as can be heard on a bus.