Simplify the following text for a Grade 11 native English-speaking person.
Original: On 1 September 1967 , the consultants submitted the Hong Kong Mass Transport Study to the government , which recommended the construction of a 40 m rapid-transit rail system in Hong Kong .
Simplified: The consultants released the Hong Kong Mass Transport Study in September 1967 , which proposed the construction of a mass transport underground railway system in Hong Kong .