The original railroad bridge mentioned above was built by Flagler for the Jacksonville, St. Augustine & Indian River Railway, which became the Florida East Coast in 1895. The first Acosta Bridge was built so high because it had to clear the open span of the railroad bridge below it. (I'm guessing it was built beside it because of the government, since a bridge represented a potential hazard to navigation and beside the existing bridge would preserve the existing channel with minimum impedance.) The bridge was opened and closed by one of two small steam engines housed on either side of the center of the span. There were two boilers and engines so one could be out of service for maintenance. There's a picture of the bridge in the Publix on Riverside Avenue from the Jacksonville Historical Society, although the caption says the bridge was opened and closed with a manual crank mechanism. While small drawbridges were hand-cranked (the Seaboard bridge over the Trout River was), it would have been impossible to operate such a big bridge that way considering the number of times it was opened and closed every day. If you look at the picture closely, you will see the boiler houses and the twin smoke stacks above them.
The increase of traffic in the 1920s caused the FEC to double-track its main line as far as Miami (the Key West extension not needing it) and replace the bottle-neck bridge with the current structure, which is a Strauss Heel-Trunion. The trusses are Pratt, which is what the old Acosta was, too. The 1920s was about the end of that style's popularity. A Pratt truss looks like this: /|/|/|/|\|\|\|\
The Main Street-St. Elmo Acosta is a Warren truss: /|\|/|\|/|\|/|\
The diagonal members of the truss give it the strength. In a Pratt truss the vertical members are an integral part of the triangle. Many smaller Warren trusses have no vertical members: /\/\/\/\