As LNW-GW JOINT says, the original plan was for the Great Western and the London & Birmingham to share Euston, but the companies failed to reach agreement. The government of the day had decreed that railway termini should not encroach into central London, an early example of the government 'interfering' in railway affairs. It's a pity that the interference didn't extend to compelling the companies to share Euston. The history of our railways is riddled with incidents like this: Successive governments, right down to the present day, have claimed that they don't interfere with railway business, then doing exactly that, usually badly! Understandable concerns about safety in Victorian times soon extended to governments setting pay, working hours, freight charges and ticket prices. Sadly, governments failed to involve themselves positively in planning the rail network, perhaps the most appropriate place for them to wield strategic control.