Step 1: Sentence S1 introduces the main idea that Calcutta has transport problems unlike other cities.
Step 2: Sentence P naturally follows by stating the result of this problem — horrendous congestion.
Step 3: Sentence R adds detail by explaining how people use the centre of the road.
Step 4: Sentence S presents the solution to ease traffic — building an underground railway line.
Step 5: Sentence S6 concludes with the factual information about the foundation stone being laid in 1972.