That would mean that airfreight is only 4 times as expensive as shipping? I'd expect it to be at least a magnitude more expensive, if not more.

I guess half as much as airfreight means that it's still considerably more expensive than shipping. But for 14 days vs 30-45 days it might well be worth it.

That depends on the shipped goods, i think? One calculation was that of a 15 inch notebook at 2kg weight, they wrote: 28 days per ship, at 2,50 € and 184 kg co². By air 3 to 4 days at 23 € and 4950 kg co² emission. By train 17 to 18 days at 5,50 € and 270 kg co². But that is for a laptop, packaged in a specific volume with that weight. They also ship tires, automobile parts, shoes, clothing, textiles, whatnot.... in both directions btw. BMW ships parts from .de to .cn

Yep, air freight is generally by weight, shipping by volume. Electronics are already sent via air, saving $3 shipping on a $1500 item doesn't make much sense. Taking the UK as an example, 70% of non-EU exports by value go via Heathrow. Would expect the value to be similar for China. By volume it's probably low single digits for airfreight, by value likely >50%.

Low value items and very heavy items are currently not viable via airfreight, that's probably where trains can be profitable. If you produce clothes your only options are to either do so in Europe (there's a lot of production in Turkey) and have it close by, or produce in Asia and wait 40 days for every order. Cutting that time for a premium could make a lot of economic sense.

Yes, I was thinking this would be useful for fashion.

If you suddenly find a particular style/colour of item is popular and selling out it might make sense to get more items shipped via train and have them arrive in 2 weeks.

If you have to put it on a boat and wait 6 weeks you may have missed your chance to sell that item (e.g., people are moving on from summer to autumn clothing, etc.).