Complex sentence: The tunnels were known to train crews as the "hell holes" as they were a very narrow bore and became claustrophobic and sooty as trains passed through and were too narrow and very unsuitable for electrification and were closed in 1953 when Woodhead 3 was completed.
Split into simpler sentences: The tunnels were known to train crews as the "hell holes" as they were a very narrow bore and became claustrophobic and sooty as trains passed through. They were too narrow and very unsuitable for electrification and were closed in 1953 when Woodhead 3 was completed.