Complex sentence: In 1925 the "inner circle" service was ten trains an hour in each direction, but this frequency of service was causing problems and a reduction to eight trains an hour would leave the Kensington High Street to Edgware Road section with too few trains.
Split into simpler sentences: In 1925 the "inner circle" service was ten trains per hour in each direction, but this frequency of service caused problems. A reduction to eight would leave the Kensington High Street to Edgware Road section with too few trains.