Relock means a new interlocking to control an existing track and signalling layout, with varying degrees of retention of existing trackside signalling equipment applying. It is usually carried out in conjunction with recontrol which is where the controlling signal box for the particular interlocking area is migrated to another place, usually by centralisation at one of the new ROCs. The techniques allow existing track and signalling layouts to remain, rather than the blank slate approach of full resignalling. Relock and recontrol often occurs as a precursor to more substantial layout changes. For example at Reading new interlockings and control screens at Thames Valley Signalling Centre (Didcot) took over control of the entire station area prior to the major remodelling activity, because the new platforms were built over the site of the former panel signal box and station interlocking, and because the subsequent complex multistage layout alterations were much more practical to manage with data changes in new computer based interlockings than they would have been using hardwired relay installations.
