According to my POV, these suggestions are needed to polish the game
-Save Game dialogbox should appear in the center of the screen
-Save game dialog box should show list of all saves so that user can overwrite and load games direcetly within the game
-In a depot, user should be allowed to select multiple vehicles and set a line to the selected vehicles
-Dragging a track adjacent to a station track should upgrade that station with one more track.
-Free placement of terminals and stations. (Sometimes they simply snap to the nearest street, may be pressing shift key while moving the mouse enables free movement)
-Rotate stations with middle mouse button