On HSR trains there is usually a rack above the seat for carryon size luggage. For anything larger your options are:

Wedge it in the space behind the last row in a first class car. There is a big space behind the last row

Leave it in the entry area of the car near the doors, there is space there.

Keep you baggage in the aisle next to you

Can be a brutal experience if you are using stations/platforms with no escalators. Porters are not readily available though they do exist in some stations.