We will be taking D and G trains between Shanghai-Nanjing-Suzhou-Hangzhou.
I will try to avoid rush hours for traveling, aiming for trains either late morning or mid afternoon.

If the luggage is left in the entry area of the cars are there racks for it or does it just pile up in the aisles? Should I lock the luggage so no one rifles through it? Has there ever been problems with another passenger walking off with someone else's luggage, stealing it, if it is out of sight?

When the train pulls into a station that's not the end of the line, how long do you have to get your luggage down, make your way to the door, and get off the train? Is it like the Shinkansen in Japan, where the trains only stop for a minute and you have to jump on and off really fast? Does one want to make their way to the door 5-10 minutes before the train stops?