PURPOSE: To increase the strength of ballast layer by a method in which a stabilizer composed of a water-expanded urethane resin solution containing an adequate amount of coal tar is spread over the surface of ballast in such a way as to enable the stabilizer to smoothly disperse and impregnate into the ballast.

CONSTITUTION: An one-component hardening type urethane resin is diluted with a hydrophilic organic solvent and mixed with water as a blowing agent to prepare a water-expanded urethane resin solution. The urethane solution as a stabilizer is spread over the surface of ballast to cause it to impregnate into the ballast layer, expand, and harden with an adequate speed. Coal tar is also added to the stabilizer to provide the stabilizer with a thixotropic property for increasing the space packing rate and strength of the ballast layer and also for controlling the hardening speed of the urethane resin in the ballast layer in wetted state. Also, as needed, a hardening accelerator is used in combination.