the junction, and this is not ameliorated by reverting to a double junction. Diamond and wide centres A double junction with a diamond can have its speed limit raised if the track centres are widened from say 4 m to say 12 m to allow for a fixed coarse-angled diamond crossing (say 1 in 8.25) with fine-angled turnouts (1 in 15). If the legs of the coarse crossing X are straight and flat, then this arrangement eliminates the need for switched diamonds and their inconvenient moving parts. SPAD protection Essentially the same as for a double junction with diamond. Scissors crossovers Fast Speed The Channel Tunnel has at its entrances a scissors crossover implemented with two turnouts replacing the usual diamond crossing in the middle. This is only possible because of the wide track centres caused by the wide spacing of the entrances to the tunnel. The result is high speeds on the turnout routes through those turnouts and minimum wear and tear. All the turnouts, P, Q, R, S, X1 and X2 are all the same high speed. Track centres need to be at least twice the normal track centres as if there was a hidden track in between. All the turnouts are identical except for handedness. Medium Speed Were this junction be made with a Diamond crossing in the middle, speeds through the turnout would be much reduced, and wear and tear on the diamond would be high. Very slow Speed At Hornsby the site is very constrained and speeds through the diamond crossing was only 8km/h, increased to 15km/h after a few changes. The site does not permit any further increase, and the restrictions dating from 1890 remain. Track centres are 12' (3.66m) throughout. Colour coding is as follows: Black - unchanged Red - scissors crossovers Blue - roads and road overbridges Purple - other rail tracks. Speed limits on the straight through the scissors crossovers are 40km/h. on the curves through the diamond crossing "X" are now 15km/h. on the curves from S - P2 and P1 to P are 40km/h. Grade separated Flyovers A double junction can be grade separated so that there is no flat crossing, reducing conflicts and reducing congestion. Flyovers require a lot of space both lengthwise and crosswise, and cannot always be built. Flying junction example at Aynho Junction. Diving junctions such as at Chatswood are a variant. Weaver Junction is