Editing Interlocking (section)

=== Electronic interlocking ===
[[File:Antwerpen Noord seinhuis.jpg|thumb|Computer-based controls for a modern electronic interlocking]]

Modern interlockings (those installed since the late 1980s) are generally [[solid state (electronics)|solid state]], where the wired networks of relays are replaced by software logic running on special-purpose control hardware.<ref name="Solomon 2003"/>{{rp|84}} The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring. In many implementations, this [[safety critical|vital]] logic is stored as [[firmware]] or in [[Read-only memory|ROM]] that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements. As display technology improved, the hard wired physical devices could be updated with [[visual display unit]]s (computer monitors), which allowed changes in field equipment be represented to the signaller without any hardware modifications.

"[[Solid State Interlocking]]" (SSI) is the brand name in trade of work of the first generation [[microprocessor]]-based interlocking developed in the 1980s by [[British Rail]], GEC-General Signal and [[Westinghouse Rail Systems|Westinghouse Signals Ltd]] in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI),<ref>{{cite report|last=Woolford|first=Paul|title=Glossary of Signalling Terms|url=http://author.rssb.co.uk/rgs/standards/GKGN0802%20Iss%201.pdf|access-date=20 April 2016|date=April 2004|publisher=Rail Safety and Standards Board|location=London|series=Railway Group Guidance Note GK/GN0802|url-status=dead|archive-url=https://web.archive.org/web/20160508205546/http://author.rssb.co.uk/rgs/standards/GKGN0802%20Iss%201.pdf|archive-date=8 May 2016}}</ref> of which [[Vital Processor Interlocking|VPI]] (trademark of [[General Railway Signal]], now Alstom), [[MicroLok]] (trademark of [[Union Switch & Signal]], now [[Hitachi Rail STS]]), [[Westlock Interlocking|Westlock]] and [[Westrace]] (trademarks of [[Invensys Rail Group|Invensys Rail]], now Siemens), [[Smartlock_(interlocking)|Smartlock]]<ref>{{cite web|url=http://www.alstom.com/products-services/product-catalogue/rail-systems/signalling/products/smartlock-interlocking-products/|title=Smartlock Interlocking|website=www.alstom.com|access-date=4 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171008205925/http://www.alstom.com/products-services/product-catalogue/rail-systems/signalling/products/smartlock-interlocking-products/|archive-date=8 October 2017}}</ref> (trademark of [[Alstom]]), and [[EBI Lock]] (trademark of [[Bombardier Transportation|Bombardier]]) are examples.