The first locomotive engine shed was built in 1837 on the up side (leading into London). The Roundhouse, 160 ft in diameter, was built as an engine shed and turntable for goods locomotives in 1846-1847, also on the up side. It had become necessary because the down passenger trains out of Euston now passed at speed through the station, which was no longer the locomotive terminus of the line. One of the first examples of a truly circular engine shed (the first, dating from 1837, was at the Birmingham Curzon Street terminus of the L&BR), the structural form was influenced by the limited space on the north side of the yard.
The building could hold 23 engines, one between each pair of columns, with the 24th track left clear for entrance and exit. In the centre was a turntable 36 ft in diameter on to which the engines had to run to be turned into their berths. Below were brick vaults, founded at natural ground level, which raised the turntable and engine storage bays 15 feet (4.5 m) to the railway level.
In 1854 the L&NWR decided on its abandonment as a locomotive depot, as it lay in the way of the strategic replanning of the northern side of the Goods Yard (and probably also because locomotives were now running longer distances between maintenance stops). It was briefly a corn and potato store, and from 1869 to 1963 became a bonded store for Gilbeys’ wines and spirits.