Grammar Rule (Superlative Degree): The sentence is comparing "The Tube" against the entire transport network of London. When comparing one thing to all others in its group (three or more), we must use the superlative degree.
The Structure: For long adjectives like "expensive," the superlative is formed by adding "the most" before the word.
Why others are incorrect:
expensive (Positive): Used to describe one thing without comparison (e.g., "The Tube is expensive").
the expensive: Grammatically incomplete; the word "most" is required for comparison.
more expensive (Comparative): Used only when comparing two specific things (e.g., "The Tube is more expensive than the bus").
Sentence Flow: "The underground railway network... is said to be the most expensive transport in London."