Railroad rails evolved from simple wooden stringers used by horse-drawn and gravity-operated tramways into wrought-iron straps laid atop wood, then cast iron rails, and finally steel rails, while their shape also changed over time. Because rails automatically guide vehicles, they remove the need for steering and make movement more predictable and easier to control. The forerunner of the modern T-section rail first appeared on the Camden and Amboy Railroad in 1830 alongside the John Bull steam engine and other early U.S. locomotives. Rail is designated by weight in pounds per yard, and by the time Western Pacific began construction in 1907, heavy main lines used 85- to 100-pound rail, while industries, spurs, and short-line railroads used rail as light as 40 pounds, most of it laid in 39-foot lengths bolted together. Today, the most common size is 136-pound rail, though rail as large as 152 pounds has sometimes been used. Since the mid-1980s, most railroads have placed welded rail, a continuous ribbon of rail thousands of feet long, on heavy-traffic lines, creating a smoother ride and less wear but eliminating the classic clickety-clack sound associated with train travel. Western Pacific was an early user of trackwork innovations such as concrete ties but remained fairly conservative in its rail, and by the time of the Union Pacific merger in 1983 much of the Oakland-to-Salt Lake City main line used 115-pound bolted rail while many branches and sidings still used rail as light as 65 pounds; the yard at the Western Pacific Railroad Museum, for instance, is mostly 65-pound rail. Although the railroad explored welded rail, its widespread use on Western Pacific lines did not occur until after Union Pacific took over, and today most of the former Western Pacific main line is 136-pound welded rail.