been opened to the public a year earlier. As the first commercial passenger train trundled over the bridge, the two countries were finally connected by railroad across the Niagara River. The successful crossings of these and later trains made Roebling's Suspension Bridge the first working suspension railway bridge in history. Engineering Roebling's bridge was supported by two limestone towers on each side of the gorge. These Egyptian-style towers stood tall on the American shore and tall on the Canadian shore. With their foundations in the earth, the limestone structures could support up to 12 million pounds (5.4 million kg). Four thick main cables held up the bridge; two cables ran through iron saddles at the top of each tower. Each cable comprised 3,059 wires that were spun with Roebling's patented technique used in his Allegheny Suspension Aqueduct. The ends of each cable were secured to cast-iron plates sunk deep in the bedrock. Support lines hung down from iron clamps that encircled the main cables, and held up the decks. Deep trusses—never before seen on a large suspension bridge—lined the sides of the bridge, and joined the two decks so that the structure looked like a cage. The trussed sides and the upper and lower decks, which spanned , formed a "hollow straight beam," reinforcing the rigidity of the bridge. The Suspension Bridge was further stiffened by guy-wires which ran from its upper deck to the top of its towers. Criticism of suspension bridges was growing after the Wheeling Suspension Bridge collapsed under strong winds in 1854. To address these concerns, Roebling added more guy-wires to secure the lower deck to the shores below. Roebling's efforts ensured that his Suspension Bridge remained standing while other suspension bridges across the Niagara River collapsed because of strong winds. Although he was not the first engineer to appreciate the need for a suspension bridge to be sufficiently rigid or to implement the methods to do so, Roebling was the first to understand the principles behind the methods and combine them in the building of a suspension bridge. Roebling proved that despite popular opinion, properly built suspension bridges can safely support the passage of heavy railway traffic. His combination of stiffening methods created the first modern suspension bridge. Such was the rigidity of the Suspension Bridge that it withstood the shockwave caused by the nearby fall of a mass of rock in 1863; the force