The conversation between human and AI assistant.
[|Human|] What were the 'Big Four' railway companies in the UK before nationalisation
[|AI|] The Big Four were London, Midland and Scotland (LMS), London and North Eastern Railway (LNER), Great Western Railway (GWR) and Southern Railway (SR)
[|Human|] 
What were the 'Big Four' railway companies in the UK before nationalisation