Complex sentence: Historically it was the Metropolitan Railway's intention to extend its Watford branch onwards into central Watford via a tunnel under Cassiobury Park and the station was constructed at a lower level in preparation for a cut-and-cover tunnel across the park.
Split into simpler sentences: Historically it was the Metropolitan Railway's intention to extend its Watford branch onwards into central Watford via a tunnel under Cassiobury Park. Watford Metropolitan Line station was constructed at a lower level in preparation for a cut-and-cover tunnel across the park.