The goal of infrastructure is not to make money directly. High speed railway, just like military, doesn't need to make money. The government should subside it like how Beijing government subsides public transportation. 300 billion is peanut money compared to its potential economic, strategic impact and how much time it saves for billions of travellers.
HSR is a good learning experience for China in a lot of areas. In addition to engineering related fields like system engineering, communication system, management etc, there's never an engineering project in Chinese history that is under such public scrutiny. Last years incidents that kills 35 people would've never set public outrage if it had happened in 20 years ago. (Strictly speaking, the incident is on D-trains, not the latest HSR). 6 months later, a very comprehensive incident report was released to public and 55 high rank officials get sacked. All these set the first examples in Chinese history.
The Japanese column writer 加藤嘉一 once wrote a article talking about the extreme similarities between Chinese HSR and Japanese HSR in early days, in both case the bosses were bold, adventurous and both later go sacked from financial troubles and corruption scandals. As a developing country, China will experience growing pains and learning curves. There will be setbacks, financial troubles, corruptions etc, but the project as a whole is extremely successful in my opinion. I once watched a video about the construction of HSR, all wiring ends will have technician's name tag attached, this gives a sense of both pride and accountability.
There was a news yesterday that Kazakhstan exports wheat to Malaysia through China's Ningbo city in the east coast, which means a cross-China transportation. Geographically it doesn't make any sense but excellent infrastructure in China makes it economically much more appealing compared to other closer routes. Eurasia continental bridge(gateway) is definitely not a dream.