can the [RR owning] the Steamboat Company move the Steamboat marker after 6.2D?
Only if it *purchases* it after paying revenues. That's what "after being purchased" is trying to say.
Otherwise, as stated in the DTG Rules on page 2: "The Steamboat Company ($40 price, $10 income) allows its purchasing corporation, *before running its routes*, to place or shift the $20 port token to any port space." (emphasis added)
The emphasized point got dropped from the GMT rules. I will correct this in the next rules version. The intended phrasing is "Upon being purchased or before running routes, the corporation owning [...]"
This has been discussed at great length in another thread.