we are holding only a single Chinese entry-visa...however want to travel to HK as well

We travel from Shanghai per train to Hong Kong, holding only a single entry-entry permit for China ... how to get from HK back to Shanghai, where we will take 48 hours later our flight back to Austria?
There is the possibility for a 72 hours free transit, but the transit needs to happen at the airport only.
However, the 144 hours free transit allows to arrive also per train at train-station in Shanghai - then start the transit - and take 1 day later the plane to Austria. That means with our single entry permit it should work to visit HK ..... Am I right?
Thanks, Joseph / Austria