Complex sentence: A number of very early British railway companies were short-lived, and were soon amalgamated into larger organisations, which were themselves grouped into the'Big Four' in 1923 (see List of railway companies involved in the 1923 grouping).
Split into simpler sentences: A number of very early British railway companies were short-lived. Many were amalgamated into larger organisations, which were themselves grouped into the'Big Four' in 1923 (see List of railway companies involved in the 1923 grouping).