Prior to the nationalisation of the railways in 1948, Britain's rail network was operated almost exclusively by four private companies. The 'Big Four' as they were called - the Great Western, the Southern, the London Midland & Scottish and the London & North Eastern - were not only nationalised in 1948, but consolidated into one large concern: British Railways. Each of the Big Four had built up its own system of working in its own geographic area with its own rolling stock, staff and livery. Thus, BR inherited a diverse mix, not only of physical plant, but of traditions and loyalties developed over generations. Additionally, management had to grapple with many and varied constraints in its desire to improve efficiency and create a nationally recognisable system. Also, cash was in short supply and much of the existing equipment was old, run down and in urgent need of attention. Further, all the major railway companies had a large number of restrictions as to which engines and stock could go where, even on their own system. Axle loading was often the deciding consideration and this governed which engine types could run on specific lines over which bridges and at what speed. For example, LNER Pacifics were banned entirely from East Anglia. Also, loading gauges differed on the national infrastructure. All these considerations impinged on BR's desire to introduce a modern range of steam engines of its own, so that these would have the widest route availability. This, by and large, they successfully achieved, though in later years even the new BR diesels had more restrictions placed upon them than was originally envisaged. The Unusual and the Unexpected on British Railways: A Chronology of Unlikely Events 1948-1968 is an assiduous and personal trawl on how BR overcome such engineering incompatibilities and bureaucratic confusion on a national scale. This engaging tribute is a historical and rail engineering document, which despite plans and intentions to unite the country with a single operating network, shows how daunting such a restructuring was.