The train is cool, but at just 24 hours is probably shy of what you’re after. If you do take it, I’d suggest grabbing it from Shenzhen. You can book online with China Travel, or a subsidiary. I’ve done that and they delivered the tickets to my office in HK.
Do not go by road. While the risk of Uigher nutjobs detonating the bus are low, the risk on the accident side and protracted traffic jams are high.
Take the slow boat to China. There’s a few sites devoted to catching freighters, here’s one:.
There used to be an old passenger boat from HK to Shanghai but that stopped around ten years ago. I took it in 1992 and it was hit by a severe typhoon on the first morning of our three day cruise, mercilessly delayed for another day and a half at sea as we went the long way around Taiwan to avoid the worst of it, and turning into a vast vomit bucket for the other 300 or so passengers.
Chief among the highlights was ordering a coffee and watching the smell of it tip a nice young crew lass over the edge forcing her to bow her head and spew into the sink just as she passed me the cup. It was a very deft move and highlighted her high level of professionalism, in my book.
It was definitely memorable!