porous portal

English

Noun

porous portal (plural porous portals)

1. (rail transport) A type of extended and perforated tunnel portal on high-speed railways, designed to reduce the build-up of pressure when a train enters a tunnel at high speed.
   • 2024 March 20, Chris Howe, “High speed underneath the Chilterns...”, in RAIL, number 1005, page 33:

     The Down line portal will measure 220 metres in length, while the Up line portal will be 135 metres long. Known as 'porous portals', they will (as the name suggests) be punctuated with holes. These holes or vents slow the build-up of pressure created by trains within the tunnels. The resulting pressure wave, if left unchecked, can produce a phenomenon known as 'tunnel boom', which is created by the piston effect. Porous portals have been used on other high-speed rail networks in Japan and France, but this is the first time they are being used in the UK.