Question

1. You have to take the bus number two.
2. You have to take the number two bus.
3. You have to take the Bus Number Two.
4. You have to take the Number Two Bus.
[Which one is grammatical?]

Answer 1

All are proper grammar. We would use #1 and #4, but "the bus" does not have to be capitalized, It's not a proper name. (In my city, the Number One bus passes my house. It's called the Number One California bus [it runs on California Street]. The designation "Number One" may be capitalized if it is the official designation of the bus line.)