I echo Rod222's comments ... PostmasterGS has certainly made a silk purse from the sow's ear I gave him!
I had not at all appreciated the significance of the cancel reading "Kufst-Mchen" rather than the other way around. It must indeed have travelled North from Italy, through Austria to the border and thence up to Rosenheim to join the main train line. It is a great pity, as others have mentioned, that I cannot supply you with an address for it is completely missing, but everything points to it being in Germany somewhere because the letter was processed on that train.
PostmasterGS, is your information about the ship route and dates from a published work or from a website? Either way I am grateful for everyone's help and I hope in future to be able to pull off some Sherlock Holmes feats myself if I get to grips with enough of the reference material.
The link given by Rod222 shows a lovely picture of the ship, Kaiser Wilhelm II, renamed Hohenzollern from 1900 to 1908 when it was decommissioned.
