The high-voltage third rail that runs next to a set of train tracks is, of course, something to be avoided. The Third Rail System for the iPhone 4, however, is to be welcomed.

This friendly breed of Third Rail combines a skin-tight phone case and a surprisingly light battery pack you attach to your phone when you need a recharge.

We’ve seen charging cases before, but most have problems. Powered phone cases usually add too much extra heft and bulk to a cell phone. If you need extra battery life, you have to turn your svelte, five-ounce iPhone 4 into a pocket-stretching doorstop that adds a half inch to the phone’s length and almost doubles the weight.

Third Rail’s system is comprised of two parts — a bantamweight (0.8 ounce) black case that’s slim and comfortable, and a backup battery that snaps onto the case, but only when the phone needs extra juice.

As a bonus, the battery can do more than power up a flagging iPhone. With its pair of mini-USB charging ports (and included cables) it can charge multiple devices simultaneously. The Third Rail battery can also be piggybacked, so you can stack additional batteries on top of the primary battery for more power.

There are trade-offs, of course. For starters, you’re stuck having to sync the phone only with its included USB cable. Also, if you have any iPhone-ready music-playback appliances, you need to remove the case to use them. I ultimately found this to be a minor distraction, since the phone slides in and out of the two-part case pretty easily.