Even the best built layout can suffer from occasional poor contact between rail joints or voltage drops.
The TKM1 can give a quick indication of the presence of volts on the rails, and for DC layouts, one of the two leds will light to indicate which rail is positive. There is an arrow next to the lesd to show which was the train will run.
Simply hold the tester against the track (fit Z, TT, N, HO and OO tracks) and look for the leds lighting.
On DCC layouts, both of the leds will light.
There is a hole so you can store the TKM1 on your key-ring, but it is probably best to remove it before testing to avoid any accidental short circuits.