Population is a double-blade sword when it comes to economy development. It brings consumption but also creates burden and cost.

China economy is becoming a puzzle as it is intertwinded with so many other factors. In 10 or 15 years, there is likely a brand-new economics theory introduced upon observing/understanding the decades-long Chinese economy.

Infrastructure building sometimes is viewed as life-support(or short term stimulus) for a deflated economy growth, and often it involves unwise spending or unnecessary projects. However, when there is strong need on the table, those civil-use construction projects might truly spur and sustain the economy growth, though it can easily generate another gigantic commodity bubble.

Subway system, possibly is or will be next life line of China economy. The capital would be well spent if the nation is going to build vast underground train system to accommodate the large and dense population in cities and make above ground traffic more efficient and cleaner. That's a huge project considering the number of cities that need subways (they might like more subway restaurants also) and the benefit and practical outcome an underground train system can bring.