If you put 4 regular rails down in a T shape, and you put a redstone torch or redstone block next to the junction, it doesn't switch. On the other hand, placing a lever replacing the torch and powering the lever will activate the junction.
Here is a list of other things that work:

• Pressure plates
• Buttons
• Levers
• Tripwires
• Redstone
• Redstone Chests
• Detector rails
• Repeaters
• Comparators
  • This is because all these things are placed unpowered, but redstone torches and redstone blocks are placed powered.

See Etho's LP Video: https://www.youtube.com/watch?v=Vfi8czqZo_0
7min 5sec