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.

The battery practically glides on to the back of the case. While attaching the battery gave me immediate juice to continue phone calls and use of my apps, with non-use of the phone, the Third Rail topped off 50 percent of the iPhone battery in a couple of hours. After the phone went from 50 percent remaining to 100 percent, the Third Rail was practically drained. The Third Rail has a power gauge, but it’s only five LED lights, so it’s not as precise as I’d like. But I got a good enough estimate at what the auxiliary battery’s juice levels were like at the start and finish of the charging.

Recharging the Third Rail’s battery for its next use took less than two hours. Then, using the included USB cable, I filled my half-depleted iPod Nano with Third Rail juice in 55 minutes.

There’s another popular case out there with a snap-in battery, the myPower case from Tekkeon, but it doesn’t let you charge additional devices or piggyback the batteries. So, I can safely say the Third Rail System is unmatched when it comes to weight, ease of use, and versatility.

But all of this does not come cheaply: The case alone is $40, and each battery is $60. Purchased together, the set is $90. But after using it just a few times, I’d recommend you get one. If you’re short on cash, maybe just, um… charge it?

WIRED Innovative iPhone 4 case that’s protective, but thin and light. Detachable battery recharges the phone as well as other devices via a mini-USB port. Eliminates the need for the large and heavy all-in-one power cases.

TIRED Proprietary case designs requires use of its own USB cable for connected charging and syncing. Convenience comes with a high price tag.

Photos courtesy of Third Rail Mobility