Blockstream's liquid sidechain, created especially for high-volume traders and exchanges to use as a high-liquidity network between each other, officially launched in Beta today and has 27 member exchanges signed up already.
https://blockstream.com/2018/10/10/liquid-launch.html
According to Blockstream CEO Back, they're already representing 50%-60% of the Bitcoin TX volume with these exchanges.
https://twitter.com/notgrubles/status/1050095663302758402
It's free to get involved as a beta node user, but there isn't much documentation yet on how to get involved as a trader.