North London line then it was switched to . Later, it was cut back to . A tunnel, the Hampstead Heath tunnel, runs under Hampstead between and . The line is double track throughout, with a mix of triple and quadruple track between and . The former North Woolwich branch included a section of single track between Custom House and North Woolwich stations, and the Broad Street branch was at one time formed of quadruple track. During the February–May 2010 blockade, the , and stations were rebuilt to allow the extended East London line to serve Highbury & Islington on fully segregated tracks
Double beta decay or double beta decay are predicted to be very long; hence, quadruple beta decay is unlikely to be realized. The eight candidate nuclei for quadruple beta decay include Zr, Xe, and Nd capable of quadruple beta-minus decay, and Xe, Ba, Gd, and Dy capable of quadruple beta-plus decay or electron capture. In theory, quadruple beta decay may be experimentally observable in three of these nuclei, with the most promising candidate being Nd. Double beta decay In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa,