Research conclusions: The system make full USe of the terrain and high—definition digital images and other basic geographic information resources provided by the Digital Earth Software to establish a three—dimensional alignment platform and it has been successfully applied in many projects，such as Ankang—Zhangjiajie and Zhengzhou—Wanzhou railways．The applications show that the system has following features：(1)The real—time digital terrain and image data are picked up from Google Earth．(2)The planar alignment is realized in Google Earth with KML technology of Google Earth，which Goosle Earth becoming a digital earth—based three—dimensional spatial platform for railway route selection．(3)A logging and query system is established for the information on the railways and natural environment along the railway．(4)The plane，vertical section，cross—sectional design models for a new railway line arc established under the traditional AutoCAD platform，which can meet various diamonds of the fine design railway to overcome the problem of optimization of vertical section．(5)The three—dimensional visualization of railway design outcome is realized in Google Earth platform．