Class 157 was the designation applied to a range of Diesel multiple unit

Diesel multiple unit

A diesel multiple unit or DMU is a multiple unit train consisting of multiple carriages powered by one or more on-board diesel engines. They may also be referred to as a railcar or railmotor, depending on country.-Design:...

 trains of the Sprinter

Sprinter (train)

The Sprinter is a family of diesel multiple unit trains in use on the UK railway system. They were built in the 1980s and early 1990s by BREL, Metro Cammell and Leyland. Most are based around a Cummins engine with Voith hydraulic transmission, although some class 158s have a Perkins engine...

 family planned for regional use, in particular for the Strathclyde Passenger Transport Executive. As well as the class number, British Rail

British Rail

British Railways , which from 1965 traded as British Rail, was the operator of most of the rail transport in Great Britain between 1948 and 1997. It was formed from the nationalisation of the "Big Four" British railway companies and lasted until the gradual privatisation of British Rail, in stages...

 reserved carriage numbers in the 526xx and 576xx series for these units.

Unfortunately, as the privatisation of British Rail

Privatisation of British Rail

The privatisation of British Rail was set in motion when the Conservative government enacted, on 19 January 1993, the British Coal and British Rail Act 1993 . This enabled the relevant Secretary of State to issue directions to the relevant Board...

 began, this project became unviable, and Strathclyde ended up receiving a batch of Class 170 units.