After the grouping of 1923 under the Railways Act 1921 there were four large railway companies , each dominating its own geographic area : the Great Western Railway ( GWR ) , the London , Midland and Scottish Railway ( LMS ) , the London and North Eastern Railway ( LNER ) and the Southern Railway ( SR ) .
The Big 4 were the Great Western Railway ( GWR ) , the London , Midland and Scottish Railway ( LMS ) , the London and North Eastern Railway ( LNER ) and the Southern Railway ( SR ) .