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The last HMS Greyhound (H05) was a G-class destroyer, launched in 1935 and sunk by German dive bombers in 1941 in the battle for Crete, with the loss of 100 lives.<ref>[http://hmscavalier.org.uk/casualtySearch.php?surname=&forenames=&ship=greyhound&page=1 Casuaty Casualty Search] (Retrieved 24 July 2019)</ref> An earlier HMS Greyhound, a Greyhound-class destroyer, also saw service from 1900 to 1919. A number of other Naval vessels have also carried the name.<ref name = "Wikipedia" />

D821 was bought by members of the [[Diesel Traction Group]], becoming the first main line diesel to be sold for preservation. In May 1973 D821 moved from Laira to Didcot, another first as a preserved private owner loco running on the main line under her its own power to Didcot, moving before the ban on such running was imposed. It moved again in the following year to a the private siding Kings Meadow sidings just east of Reading station, in the Reading. Between February 1977 and April 1981 the locomotive underwent an overhaul at SwindonGas Works site, and then moved to where it was a notable sight from trains passing on the North Yorkshire Moors Railwaynearby Great Western main line. She finally arrived on From 1975 it was in the company of the SVR in September 1991DTG's [[BR Class 35 D7029]].<ref name ="SVR9"/>

Between February 1977 and April 1981 the locomotive underwent an overhaul at Swindon, and then moved to the North Yorkshire Moors Railway. It finally arrived on the SVR in September 1991.<ref name ="SVR9"/> An overhaul commenced in 2014.<ref>[http://www.svr.co.uk/MeetTheLoco.aspx?id=90 SVR meet the Locomotives]</ref> After a long period under repair at Old Oak Common (OOC) she it returned to the SVR after the OOC open day in September 2017<ref>[http://www.bropendays.co.uk/oldoak2017.html BR Open Days.co.uk] (Retrieved 4 June 2023)</ref>, before returning to use in April 2018 in time for the following month's [[Diesel galas|Spring Diesel Festival]]. From 2021 it saw extensive bodywork repairs, replacement of primary suspension springs, attention to the brake system and a repaint into BR Blue with full yellow ends.<ref>SVR News 218, Summer 2022, p. 23</ref> It returned to traffic for the 2023 Spring Diesel Festival. D821 is a popular [[SVR-based diesel locomotives visiting other events|visitor to other heritage railways]].