12.Railroad Transportation

Manbuiltrailroadslongbeforehebuiltsteamenginesandbeforehehadevenmasteredthebasictechnologyofengineeringwithironandothermetals.Theearliesttrainswereconstructedoftimberandwereinoperationnotlaterthanthefourteenthcentury.Theywerebuilttoovercometheseverelimita ......（本文共 5610 字 ）     [閱讀本文] >>