This is about the fundamental capacity constraints and costs of rail versus maritime transport. The former will always be more expensive than the other, and always be further capacity constrained. The only thing that could change that is a quantum leap in several technologies, in which case the current investments become useless.
> “Rail services are considerably cheaper than air and faster than sea”
Where are you getting this quote?
This increases wear on the train and tracks. We might have materials breakthroughs that change the balance. But such materials would make (a) shipping more efficient and (b) large fractions of the existing Belt and Road obsolete.
> right now the China-Belgium trip can take 20 days by train (cf. https://www.train-chain.com/) and usually around 30 days by sea
Emphasis on can. In any case, there is a reason the world's commercial shippers are all focussing on Arctic maritime routes.
Rail is a neat way of controlling territory. It's better than trucking or flying, when it comes to cost. But water beats it at scale.
Yes. But it's also much faster. This creates new opportunities. Rail freight is not replacing sea or air freight, it's supplementing it.
Expensive but faster freight means that heavy but valuable items can be delivered faster. Heavy Air freight (>100 kg) typically costs 15-20 percent of the value of the goods. Rail wright can be expected to cost 5-10 percent easily, more for items weighing more than 1000 kg. That's value created by the rail.