If you don't have it copied any other way, the reference to the contents of a control on an ACTIVE form would be
Forms("formname").Controls("ControlName")
And when you use the syntax above, you are using the default for the item selected, which is .Value - so you might use
"UPDATE [Dummy Table] SET [Line Rating] = '" & Forms("formname").Controls("controlname") & "' WHERE [Sub Station] = 'crosby'"
The trick is, of course, to use the ' marks to quote text so that Access doesn't try to translate the resultant string, and you use & marks to force concatenation of the value in question.
Finally, if you are in a context where Me actually works and the variable name Rating exists, using Me.Rating is not necessary because (just plain) Rating is adequate. And if you are calling a "common" subroutine where Me does NOT work, Me.Rating is undefined.
Personally, I would make it a two step process by putting a ".LostFocus" routine behind the control in question and have it store the value in your Rating variable, complete with any mode conversions and editing as required. Like, for instance, if it is a string you might wish to trim the string.... Rating = Trim$([controlname])
After that event has fired, your rating is available until you go change it and tab out of the control again.