Porterbrook is a British rolling stock company (ROSCO), created as part of the privatisation of British Rail. Together with Angel Trains and Eversholt Rail Group, it is one of the three original ROSCOs.

Porterbrook was established in March 1994 and was privatised via a management buyout during November 1995. In August 1997, it was acquired by the transport specialist Stagecoach at a far higher value. Two and a half years later, Stagecoach sold Porterbrook to the British banking group Abbey National. Prior to this, Porterbrook had entered the freight leasing market for the first time as well as the continental European rail leasing market; the latter activities were sold off in December 2004, resulting in the creation of CB Rail. On 27 October 2008, Porterbrook was purchased from Abbey National by a consortium of Antin Infrastructure Partners, Deutsche Bank and Lloyds TSB.

During the 2010s, Porterbrook engaged in various projects to overhaul or dispose of aging rolling stock. Some of its Class 319 electric multiple units were rebuilt as Class 769 bi-mode units to work on both electrified and non-electrified routes. In 2020, Porterbrook started a £55 million five-year programme to refurbish 214 Class 377 'Electrostar' EMUs operated by Southern. Furthermore, many of its Class 43 power cars, historically operated as a part of the Intercity 125, were sold or donated to other parties, along with its fleet of Class 90 electric locomotives.

Porterbrook was established on 21 March 1994 as a subsidiary of British Rail in preparation for the privatisation of British Rail. It was named after the Porter Brook, a river that passed by British Rail's Sheffield offices (Specifically Sheaf House, adjacent to Sheffield railway station, which was demolished in late 2005 as part of a city centre regeneration project). In November 1995, Porterbrook was privatised via a management buyout.

In 1996, Porterbrook entered the freight leasing market though a deal to purchase and lease back 70 locomotives and 699 wagons from Freightliner. On 28 August 1996, the company was purchased by the transport conglomerate Stagecoach in exchange for £825 million; this deal was subject to considerable criticism for the low value returned to the taxpayer. On 20 April 2000, Stagecoach opted to sell Porterbrook to the British banking group Abbey National for a reported £1.44 billion.

Porterbrook established its own subsidiary to handle its European rail leasing activities; this was sold to a joint venture between Babcock & Brown and Bank of Scotland Corporate Europe during December 2004, creating CB Rail. At the time of the takeover, the subsidiary had leasing arrangements for 28 locomotives and 67 coaches.

On 27 October 2008, the business was purchased from Abbey National by a consortium of Antin Infrastructure Partners, Deutsche Bank and Lloyds TSB. The sale, which reportedly valued Porterbrook and its 6,014-strong rolling stock inventory at £2 billion, gave rise to concerns that Abbey National, and its Spanish parent Banco Santander, were subject to considerable financial stress at that time.

During the 2010s, Porterbrook engaged in various efforts to overhaul and refurbish elements of its rolling stock, particularly multiple units. One proposal was the refurbishment of its British Rail Class 144 'Pacer' diesel multiple units, referred to as the Class 144e (Evolution), it would bring it up to the requirements of the Persons with reduced mobility-Technical Specifications for Interoperability accessibility regulations. It featured several upgrades such as the addition of new 2+2 style seating, a fully accessible toilet, two wheelchair spaces and spaces for bicycles and luggage, as well as Wi-Fi and media screens.