Circle Lines
Build circle lines whenever you can.
Redo Lines
Pause the game and redo lines into smaller units when a line gets too long.
Upgrades
Keep the lines in order and take advantage of the upgrades which come in the form of budget increases which will feature new lines, platforms, capacity, and much more.
Tunnels
Get tunnels whenever possible, otherwise go for carriages.
Collapsing Station
If things get tight, you can always move one train to the collapsing station (it finishes the stop at the next station and then respawns where you placed it).
Rare Symbols
Connect rare symbols to at least 2 lines.
Variety of Shapes
Have a variety of shapes on each main line (with at least one square, triangle and circle) but also minimise distance travelled.
Interchanges
Make rarer shapes (squares, pluses etc.) interchanges or have them close to interchanges.
Early Rails
Think about how you place your early rails thoroughly. They can have a significant impact as more stations appear.
Extra Trains
You can drag your extra trains between rails, so watch for crowded stations and adjust accordingly.
Passengers
Passengers can be unloaded at any same-shaped station, keep that in mind when creating your railways.
Stations
Try to avoid having two or more stations in a row because when a train is full when emptying the first station, it will go past the second one as it has no place to drop its passengers. The best way to deal with this is to break the cycle by adding a station from a different type inbetween, for example a triangle.
Moving Carriages
You will know when you are moving a carriage when the object you are dragging is a perfect rectangle.