The King's Cross train isn't always as crowded as Albie has said. At certain times...commute times and in the late evenings (2300-ish) it can be busy, but I've been on that particular train dozens and dozens of times when there have only been 6-9 people in the car total.
The train is about a 90 minute journey. Realize there are actually TWO different trains that alternately run on the same track. There is a "commuter express" train that runs the same route in about 60 minutes, and then the normal train that makes more stops and takes 90-ish minutes.