When railroad lines run over swampy or unstable ground, they are sometimes supported by layers of timber and smaller pieces of wood that form a sort of raft on which the ballast and tracks are placed.

• 铁道线路过沼泽或者土壤不稳定的地带时，人们有时还用原木和小块的木材修成木筏一样的结构，再在上面铺道碴和轨道。