The primary purpose of railway bridges is to safely and reliably carry freight and passenger train traffic within the railroad operating environment. The majority of the railway bridges in the North American railway infrastructure have steel superstructures. Planning of railway bridges involves the careful consideration and balancing of multifaceted, and often competing, construction economics, business, public, and technical requirements. The economics of a bridge crossing depends on the relative costs of foundations, substructures, and superstructures. Most new freight railway bridges are constructed on existing routes on the same alignment. Construction methods that minimize the interference to normal rail, road, and marine traffic enable simple erection and are cost-effective, which must be carefully considered during the planning process. Site conditions are of critical importance in the determination of location, form, type, length, height, and estimated cost of railway bridges. Erection procedures typically depend primarily on site conditions, contractor experience, and equipment availability.