First open a simulator:
open /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app
Then open a given URL in mobile Safari:
/Applications/Xcode.app/Contents/Developer/usr/bin/simctl openurl `/Applications/Xcode.app/Contents/Developer/usr/bin/simctl list | grep Booted | perl -ne'/\(([A-Z,0-9,-]+)\)/ && print "$1"'` "http://localhost:3000"