The function of the "transit" button seems a little weird generally (in web maps too).
My observation is that it (1) turns off some of the excessive highlighting of highways and other roads (which for some reason is done by default even in very transit-oriented cities), and (2) it turns on highlighting of "rapid transit" lines.
Unfortunately the definition of "rapid transit" used for this function is sometimes very poorly thought out.
For instance, Tokyo has a massive rail system that covers the entire metropolitan area with a fairly consistent level of service. However Google maps only considers the city-center subways—which form about 25% of the entire system—to be "rapid transit", although from a user's point of view there's very little difference between subways and most other rail lines in Tokyo (they generally use similar scheduling, rolling stock, stations, and even station spacing in many cases). So enabling the transit option in Tokyo highlights subways, but it actually de-emphasizes many other (essentially equivalent) rail lines!
The only thing I can think of is that the design of this feature was based on the particular city (e.g. NYC), and then sort of slapped onto other locations without a whole lot of actual thought... :(
My observation is that it (1) turns off some of the excessive highlighting of highways and other roads (which for some reason is done by default even in very transit-oriented cities), and (2) it turns on highlighting of "rapid transit" lines.
Unfortunately the definition of "rapid transit" used for this function is sometimes very poorly thought out.
For instance, Tokyo has a massive rail system that covers the entire metropolitan area with a fairly consistent level of service. However Google maps only considers the city-center subways—which form about 25% of the entire system—to be "rapid transit", although from a user's point of view there's very little difference between subways and most other rail lines in Tokyo (they generally use similar scheduling, rolling stock, stations, and even station spacing in many cases). So enabling the transit option in Tokyo highlights subways, but it actually de-emphasizes many other (essentially equivalent) rail lines!
The only thing I can think of is that the design of this feature was based on the particular city (e.g. NYC), and then sort of slapped onto other locations without a whole lot of actual thought... :(