I’ve advocated voltmeters on train layouts before, but I realized something, after checking out a new-to-me Lionel KW transformer: It’s very easy for a vintage transformer to deliver more voltage than you intend, and through no fault of its own.
The “problem” is that transformers step the voltage down on a percentage basis. In the 1950s when they were designed, household voltage was 110 volts. So a transformer designed to deliver a maximum of 20 volts stepped down to 20 from 110. Today, however, it’s not uncommon for the voltage at the outlet to be 115, 120, or even 125 volts. So that maximum throttle of 20 volts is now closer to 22 volts in this day and age, because you can safely assume the source voltage is 10% higher. And the voltage markers on your transformer, which never were all that accurate to begin with, will be even less accurate.
Most postwar Lionel trains are designed to run at 18-20 volts, so if you turn the throttle to the max, you’ll probably overvolt them. The situation gets worse with other makes of trains.
Marx and American Flyer trains run fine off a Lionel transformer, except that they’re designed for a maximum of around 14 volts. So it’s very easy to unintentionally overvolt those trains to 22 volts if you turn a Lionel transformer to the max. They’ll run, but they’ll soon overheat and the windings on the motor armature will burn and short out.
While one venue I won’t mention by name might advocate only using modern transformers, a more practical and sensible approach is to add a $6 AC voltmeter to your setup, to make sure you’re never delivering more than 14 volts to your trains. While you’re at it, you might add a similarly priced AC ammeter to make sure you’re not overloading your transformer either. See my earlier post for instructions on wiring them in.