Q. What is a tilebase?
A. It's one or several tiles that make up a building or structure. Tilebases can be 1x1, 2x2, 3x3, or 4x4 in size.
Q. Why can't I edit roads, powerlines, and other infrastructure?
A. Not everything in SimCity 2000 is alterable. Infastructure must remain "as-is". Pretty much everything else is alterable. I guess Maxis felt that roads look the same everwhere, as do powerlines and highways. Tsk, tsk, Maxis! Not everyone lives in California!