The old Westminster underground station served only the shallow, ‘cut and cover‘ District and Circle Lines. The arrival of the much deeper Jubilee Line extension, and the construction of the New Parliamentary Building, presented the opportunity for a complete redesign. The new station is a much bigger, more complex interchange. It was considered essential to the open, accessible character of the New Parliamentary Building that its ground floor should be just that - a ground floor at street level, not a raised podium. This, however, created a headroom problem for the District and Circle Lines: the railway lines were therefore lowered by 300 millimetres so that two full storeys could be inserted between platform level and street level. The upper storey contains the ticket hall, accessible from the Bridge Street colonnade of the New Parliamentary Building, and the lower storey contains the platforms. The railway lines cut across the site at an almost 45 degree angle and this has influenced both the plan and the structure of this part of the station. Elements such as dividing walls, escalators and ticket barriers follow either the diagonal grid of the railway line or the orthogonal grid of the building above. Round concrete columns have cruciform capitals which are linked together to form a slightly skewed pattern of rectangular and lozenge-shaped ceiling coffers. The frame is relatively light since it does not have to bear the weight of the building above, which has its own above-ground transfer structure. Below the District and Circle Lines station lies the 30-metre deep ‘escalator box‘ which provides access to the Jubilee Line platforms. Like most of the other new stations on the line, this is essentially a single volume and therefore quite different from the tangled warren of passageways characteristic of older Underground stations. The concrete diaphragm walls of the box were cast in deep trenchers before the pit was dug. The longer walls, which are unbraced by floors or walls, are stiffened by a vertical grid of beams and buttresses constructed from the top down as excavation proceeded. Further stiffening is provided by horizontal flying shores of solid steel, 600 millimetres in diameter, which connect the buttress walls to a row of concrete columns in the middle of the box. These shores form the supporting structure for the escalators, which are treated as distinct objects within the box, preserving the unity of the space. Travelling up and down the escalators reveals unexpected long views, up, down and sideways and the Piranesian effect is heightened by the finishes, or rather the lack of them. The rough concrete of the box wall remains visible, though framed by the massive grid of the beams and buttresses, and the flying shores are almost brutally simple. Even the finned outer casings of the escalators have an almost military toughness. The railway tunnels themselves pass to one side of the box, one above the other to keep them as far away as possible from the foundations of the tower of Big Ben.
