How they achieved the speed? 

Interesting mystery about 747SP. Hundreds of pages written about it, how they made it lighter, shorter, more powerful, etc. But nowhere they say what they did to increase the speed. It flew just 200 kph slower than the speed of sound. And it's a good thing. But neither Boeing themselves, nor any other manufacturer ever tried to repeat this feat. Why? With modern airliners being able to cover up to 19,000 km why nobody makes them faster? What was done to the airfoil in that regard? Does anyone know a source where this angle is covered?