Double junctions are avoided because the flat crossing of the outer turnout route over the inner through route is more maintenance intensive than two additional switches, and it includes a short length of unmaintainable plain line in the sixfoot... #RailwaysExplained https://twitter.com/shuntingwest/status/1259986071720464384

(I deleted the original version of this tweet because it turns out that signalling is an eternal black magic to me. STICK TO PWAY, DENNIS.)

But, and this is why I love twitter, there is now a string of signalling folks adding lots of useful information underneath this thread!