This is big: Our sources divulge that UK firm Grimshaw and LA-based Gruen Associates have won the commission to master plan the six million square feet of entitlements at Union Station in Los Angeles. A formal announcement is expected this coming Monday on Metro’s web site (our leak is unconfirmed), with the Metro board approving the firms after that.  Grimshaw has made a name for itself designing infrastructure and transit stations around the world, including Lower Manhattan’s upcoming Fulton Street Transit Center and London’s Waterloo Station. Gruen recently completed design on phase one of the Expo Line and has served as executive architect on several recent projects, including the Pacific Design Center. The site around Union Station encompasses about 38 acres and is anticipated to become a transit and commercial hub for the city. It will likely include offices, residences, retail, entertainment, parks and a potential high speed rail station.