The Iron Trunk (aka Cosgrove Aqueduct) is a navigable cast iron trough aqueduct that carries the Grand Union Canal over the River Great Ouse, on the borders between Buckinghamshire and Northamptonshire to the northwest of Milton Keynes.

Formed of two cast iron trough spans, with a single central masonry pier, it is the world's first wide canal cast iron trough aqueduct. The abutments were also constructed in masonry but were re-faced in brick during the 20th century. The trough is 4.6 m (15 ft) wide, 2 m (6 ft 6 in) deep and 31 m (101 ft) long. The canal surface is some 12 m (40 ft) above the surface of the river. The approach earthworks total 800 m (0.5 miles).

To cross the course of the River Great Ouse, the lowest point between the summits at Tring and Braunston, flights of locks were constructed for the Grand Junction Canal (later absorbed by the Grand Union Canal) in 1800 - four at the southeast and five at the northwest - which permitted the canal to descend to cross the river on the level. 

The company's engineer, William Jessop, later designed a three-arch brick viaduct so that the canal could cross at a higher level, reducing the water loss and delay in locking down to river level. The structure was opened in 1805, but a section of the canal embankment soon collapsed; after repairs, the aqueduct structure itself collapsed in February 1808, severing the canal. The courts subsequently determined that the contractor had been at fault and awarded damaged to the company, covering both loss of trade and the cost of a replacement aqueduct.

While another of the company's engineers, Benjamin Bevan, set to work on the design of a replacement structure, the original lock system was quickly re-instated. With Thomas Telford having successfully constructed cast iron trough aqueducts at Pontcysyllte (see entry) and elsewhere, Bevan determined to take forward the technology by designing the world's first wide canal cast iron trough. The structure was cast at the Ketley foundry at Coalbrookdale which had produced the Longden-on-Tern aqueduct for Telford.

The comonents were transported to Cosgrove by the canal itself. After assembly then erection on site, the new structure was completed in January 1811. Bevan designed the floor sections to be arched, providing the additional strength needed to resist the substantial additional loading of a wide canal. The arch ribs which are intergral to the side plates of the trough confer additional shear strength.

The towpath is cantilevered from one side, as at Pontcysyllte, and supported by diagonal struts. There is a cattle creep in the embankment on either side of the main structure to provide field-to-field access for cattle. The aqueduct was refurbished in 2011 to celebrate its bicentenary.