Complex sentence: The number of rail passengers crossing to South Wales which had been envisaged was reduced after the opening of the Severn Tunnel in 1886 linking South Gloucestershire in the west of England to Monmouthshire in south Wales under the estuary of the River Severn.
Split into simpler sentences: The number of rail passengers crossing to South Wales, which had been envisaged, was reduced after the opening of the Severn Tunnel in 1886. The tunnel linked South Gloucestershire in the west of England to Monmouthshire in south Wales, under the estuary of the River Severn.