Railroad of subways are usually constructed along narrow streets, either in tunnels or on ground surface, to avoid any settlements of the neighboring structures. Hence it includes several short radii curves. All previous railway practices agreed that running of trains on these curves resulted in excessive rail wear due to abrasion action of wheels, specially on the outer rail. This Phenomenon leads to high maintenance cost, thus this paper studies a new method for determining the suitable radii of horizontal curves used in alignment of railway tunnels regarding minimum cost.