Built by Bombardier at the Derby factory, the units are part of the Turbostar family of units which also include the class 168, 170 & 171 units. The units were delivered into service during 2010-2011 and are made up of 2 or 3 car sets, all capable of 100 Mph running. There are 4 sub class variants (as at 2014) and 2 external variant distinction. Class 172/3 & 172/4 are fitted with end gangway connection doors to allow multiple connection access. Unit numbers as first delivered were:- 172001-172008 172101-172104 172211-172222 172331-172345