Complex sentence: The growth in train overcrowding is largely attributed to increased passenger demand and the "walk-up" nature of British railways, in which seat reservations are not required, combined with the inability to run extra trains due to deficiencies in the railway signalling system.
Split into simpler sentences: The growth in train overcrowding is largely attributed to increased passenger demand and the "walk-up" nature of British railways, in which seat reservations are not required. The major problem in solving the problem of overcrowding has been the inability to run extra trains due to deficiencies in the railway signalling system.