It's possible the "SG" was from "Thomp"S"on's Sidin"G" and not "S"idin"G". Who knows any more?????
As for naming it after a siding:
Why the following?
FY for Freeport
GD for Glen Cove, Glen Street (1903)
GW for Glendale
GY for Greenport
HN for Hicksville prior to "DIVIDE"
KF for King's Highway
MY for Montauk
ND for Far Rockaway (1878 Bk of Rules)
RI for Wantagh (1903)
and on, and on, and on . . . . . Pretty unusual combinations, and perhaps reaching in some cases. Had to be a reason, but, as you said, those that named these places are long gone. Who knows where they got their calls from. Could have been a local creek nearby, or a popular business trackside at the time and the initials stuck, or a nickname for an area that EVERYONE knew at the time but fell out of use as years passed, but, the railroad being the non-conforming company it was, retained the calls of the old name . . . . .(like "F" at Mastic for "Forge"!)
And who knows . . . .maybe "SG" WAS named for the Sagtikos land grant. But, unless the fact can be confirmed through another source, we have to go with what is (or was) common knowledge at the time.
If you find anything confirming your theory in your research, PLEASE let me know and perhaps we can rewrite the "history books!" Look what Columbus accomplished!
Thanks for your insight!!!
Dave