Mass Transit Guide

To answer that, I created a graphic summarizing who uses what form of transportation.

Mass Transit Options

Bus

• Shuttle Bus: Provides a way for sims to travel within the city. Note: Bus stops lower land value, so be careful when placing in high wealth areas.
• Regional Bus: Provides a way for sims to travel between other connected cities in the region that also have bus stops.
• Park and Rides: Placing park and rides throughout your city will greatly increase the amount of Sims using your bus system. They're small and easy to fit into all the nooks and crannies of your city. Highly recommended when using buses. The park and ride can also double up as a parking garage to serve the train station, maglev, street car, shuttle bus, and ferry dock. The bus terminals contain their own garages and thus can be placed within a shopping center to best benefit the commercial zones.
• The Double Decker Buses in the UK building set bundle have a slightly higher capacity than the normal buses (100 vs 80). This makes the UK bus terminal ideal for handling a large amount of low-mid wealth tourists.

Streetcars

• Streetcar tracks can be placed in avenues or on their own. Pedestrians can also walk alongside the streetcar tracks, so you can use them for pedestrian shortcuts even if you don't have any streetcars.

Trains

• Brings in tourists. Very useful for low and medium wealth casino/tourist cities. Place within walking distance of Lower and Medium Wealth Casinos.
• Train tracks require roughly double the length of a road in order to construct bridges and tunnels. Likewise, I recommend to try and build your train tracks with 45 degree angles in mind.

Boats

• Passenger Ferries: Brings in low and medium wealth commuters from other cities in the region that also have ferry terminals.  
• Cruise Ship Terminal: Brings in medium and high wealth tourists from outside the region. Very effective method for bringing in high wealth tourists. Place within walking distance of high wealth casinos.

Airport

• Brings in tourists from outside the region. The number of tourists generated pales in comparison to the cruise ship terminal, airship terminal, and train station.

Blimps

• Airship Terminal: Very effective method to bring in medium and high wealth tourists. 
• Commuter mooring: Brings in medium and high wealth commuters from other cities in the region that also have commuter moorings.

Maglev

• Provides a way for all wealth classes to travel around a city
• Maglev stations futurize roughly 2x2 blocks (around the $505 road tiling marking). The tracks don't.
• Don't require a centralized network in order to function, meaning that you can have a variety of maglevs that go directly to specific points in a city without having all of your tracks connect together.
• Conforms to the shape of roads and the terrain, but may be independently adjusted in height regardless of what the ground is like below them. However, these tracks can't create tunnels. 

Miscellaneous Transit Items

Skybridges

• Skybridges are classified as a "transit" option and thus allow all vehicle and utility agents to transfer through them. Thus, you can use a skybridge as the means to quickly move sims from one point in the city to another as long as you have the ControlNet and money pending.
• School buses also travel along the sky bridges. I can't confirm if the commuter buses do as well.
• Skybridges are built based on the height of the skybridge station and how the stations can connect to each other. This means that a skybridge can be as long as you want it, so long as you got the money.

University Dirt Paths

• The University dirt path can be used to create pedestrian shortcuts. It doesn't need to be attached to the University, and will remain if you demolish the rest of the University

Land Value Impacts

All mass-transit options will lower adjacent land values. The bus stops, park and rides, bus terminals, ferry terminal, airship hangar, street car stops, train station, maglev stations, and streetcar station all have the same effect as a mid-tech factory in terms of how they lower land value (the very structure itself plus a very small radius around it). I often find that sims won't mind these things being built near them but not on top of them. However, this is easy to counter for you can use a variety of parks that will increase the desirability for the R and C zones.

The airport generates a massive kick-back in land value and almost nobody wants to live near it. Industrial zones, however, like the airport, and you can even use the airport as more means to ship freight for the factories.

An airship terminal, train station and regional bus station anchor my circular casino city.