2006, Carl Bro Group Ltd, FareXChange Scoping Study Final Report v1.0, section 8.6.1, Department for Transport, http://naptan.dft.gov.uk/transxchange/farexchange/FareXChange%20Scoping%20Study%20Final%20Report%20v1.0.pdf,
Particular considerations apply to circumstances such as doughnutting when fare elements do not correspond to controllable elements, so there arises the opportunity for fare evasion.
Source: wiktionary
2012, WickedWolfie, reply to South West Trains. Liars? Thieves? Generally unhelpful., MoneySavingExpert Forum, https://forums.moneysavingexpert.com/discussion/comment/57775841/#Comment_57775841,
The post before you (rightly) says that overriding season tickets is a regular fare con, as is doughnutting (having tickets for the start and the end of the journey with a gap - often large - in the middle), the latter exposes the limitations of ticket gates.....
Source: wiktionary
2014, scottwalds, Re: [Mega Hot tea] Penalty fares on trains, Tony's Non-League Forum, https://web.archive.org/web/20220120132212/https://www.nonleaguematters.co.uk/forum/gforum.cgi?post=561451#561451,
There is a technique known as doughnutting, where you buy a ticket covering your first and last stops, to allow you to pass through the ticket barriers at both ends.
Source: wiktionary