Place a sticky piston facing a slime block. put a Detector rail with a minecart on it onto the slime block. put a lever on the piston and flick it a few times. break the minecart, and you have a Powered detector rail. this doesn't always work, if it doesn't place a minecart back on and try again.

It isn't a visual glitch, you can place blocks next to the rail. if you place a redstone component it becomes powered.