Detector Rail stays active

Put the summary of the bug you're having here

What I expected to happen was:
The mine cart activates the detector rail when it rolls over it, then the mine cart rolls away and detector rail disables again.

What actually happened was...:
~_Sometimes~
-when placed the detector rail is already active.

~_Always~
-detector rail stays on even when no cart is on it or near it.

Steps to Reproduce:
1. build a short non-powered rail (any length)and place a detector rail somewhere along the line.
2. place a line of redstone dust dirctly under detector rail (perpendicular the the track) example: +
3. place a minecart (any) on track and push the cart until it rolls over and passes the detector rail....
4. (RESULT) the redstone should stay powered even after a block is updated