Here are some track direction control ideas for those who are building transportation systems. I heavily use Train Boarding Tracks in my design so that I can stop at various places in my underground rail system. Quick review of how the boarding track works:

1) The cart approaches the Boarding Track.
2) Cart stops. it waits patiently until the Boarding Track is powered.
3) Once the track is powered the cart moves on its way. I like to use buttons to power the red stone on the floor next to the Boarding Track.

Allright. Simple enough - I can stop the cart and continue. Why would I?

One thing you can now make a subway system with many stops along the route. Each subway station would allow you to stop, and then choose to either get out of the cart or continue.

Another example: You want to stop just before a switch track so that you can choose which direction.

1) You are in the cart as it approaches a Train Boarding Track. Notice in the picture that the switch track will turn to the right down the diverging line. You'll also notice that I have the button on the green wool as my trigger to contine (it will power the Boarding Track when I press the button.)
2) You stop on the Boarding Track and can choose to go right or straight (depending on the direction of the Switch Lever. You remember switch levers right?
3) You decide that you want to go straight instead of right. Right click on the lever to switch direction.
4) Now click on the button will make the cart continue.