I've recently read that the Wateroo&City Line will have new relay based interlockings at the locations where the signalling will be changed. Why is it that relay based interlockings have been chosen over more modern solid-state interlockings or computer based interlockings?
Basically the design of the W&C signalling for the blockade is not a resignalling but more of a re-working to encompass some different safety standards and remove redundant wiring. As such the scope of the work does not encompass a brand new interlocking.
Also, Westinghouse have enough work without an extra Westrace to design and Metronet can design a relay interlocking (or more accurately the modifications to the exisiting relay interlocking) in house.