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This was a single track viaduct over the River Forth with a central swing section.

Also known as Alloa Swing Bridge, the Throsk Viaduct or 'The Other Forth Bridge'.

The bridge was 1615 ft long overall. This was a 20 section bridge, a series of steel bowstring girders. Mounted in the middle of the swinging section was a control cabin above track level, this section pivoting in the middle. This was powered by a steam engine in the pivoting point, built at the St Rollox Works. Above the pivot was the control tower, a octagonal building above the swinging girder and track. The navigable part of the river was at the north of the channel. Girders on either side of the swinging section were longer within the navigable channel. The arrangement was, from the north, 4 fixed girders, a single rotating girder, and 15 fixed girders.

The engineers for the Caledonian Railway were Crouch and Hogg, and contractors for this bridge Watt and Wilson.

The last traffic over the bridge was in 1968, local railbuses from Larbert to Alloa and longer distance services with DMUs. Following this the bridge continued to swing until 1970 when it was fixed open. Up to this date coal was still delivered by rail for the steam engine. Demolition was between 1971 and 1972.

The piers still stand but girders and swing section have been removed.

The line was double track at either end, the southern approach being controlled by Throsk signal box and northern approach by Forth Bridge Signal Box. The steam engine is now at the Scottish Maritime Museum in Irvine.

The bridge was required to swing to allow shipping access upstream to Stirling. The approach was far from easy as an approaching vessel would be unaware of whether the bridge was opened or closed, the bridge being low lying and round a bend on the approach from either direction. With the current of the river and tides a vessel could not always stop and there was little time to open the bridge for its operator. Crew spoke of 'shooting' the bridge. Due to these complications regular service vessels were fitted with hinged masts and funnels.

In strong wing a vessel was blown against the bridge in 1899.

The bridge was closed between 1904 and 1905 following damage when the schooner 'Stirling', under tow, struck the bridge damaging girders on the 12th of October 1904. One pair of braced piers fell entirely and two spans fell with it. The track was left dangling over the gap.

A surrendered German warship broke its mooring and struck the bridge resulting in its closure between 1920 and 1921. Spans 10 and 11 were destroyed. The bridge was rebuilt with some piers entirely replaced and additional cross bracing added.

There was a safety boathouse on the north bank, just east of the bridge.

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