The Lionel KW transformer

The Lionel KW transformer was the second largest transformer Lionel made in the postwar era. It delivered 190 watts of power and provided two handles to control two trains. Internally, the design is very similar to the ZW. If the ZW was Lionel’s Cadillac transformer, the KW was the Buick. I always thought Lionels were overrated until I ran a 675 locomotive with a KW.

There was a time when nobody made modern transformers the size of a KW or ZW. Now that they do, the ZW and especially the KW cost a lot less. I remember when a reconditioned KW cost $200. Today you can get one for under $100. An as-is KW with minor issues will cost half that. These days, the KW is a bargain.

Lionel KW transformer
This drawing illustrates all of the key functionality of a Lionel KW transformer. Be sure to use an external circuit breaker if you use the 6V output.

Lionel made the KW for 15 years, from 1950 to 1965. The 1950-1956 model uses a different coil from the 1957-1965 model. You can tell the difference from looking at the posts. Early models provide 20 volts at posts A-D. Later models provide 19 volts at A-D. The older model can output 20 volts maximum at the throttle, while the newer model tops out at 19 volts.

Lionel KW transformer safety and repair

Always check a new-to-you transformer for safety first. KWs are very rugged, so frequently all they need are new rollers and a new power cord.

The KW opens up fairly easily. Lift the two handles straight up, then remove the four machine screws on top of the case.

In the event that a binding post is loose and dead, replacing them is pretty easy. The terminal panel they attach to floats in the case so you don’t have to do a lot of maneuvering. Either use a commercial binding post that attaches with a nut, or use an 8-32 machine screw and nut like I did on my ZW.

Running trains with the Lionel KW transformer

Connect post U or C to the #2 posts on your lockons, the outer rail. Connect post A to post #1 on the lockon (the center rail) on one loop of track and post B to post #1 to your other loop’s lockon. When you use post C for the outer rail, you get a range of 0-14 volts, which is very good for non-Lionel trains like Marx and American Flyer. When you use post U for the outer rail, you get a range of 6-20 volts with the early model, or 6-19 volts with the later model.

The ability to set a maximum voltage of 14 volts is a big advantage for Marx and Flyer trains, as their motors are designed for a maximum of 14 and 15 volts, respectively. Higher voltages will burn up their armatures.

I’ve never seen a really good explanation why a KW or ZW seems to fight voltage drop, but some research indicates a motor will pull more amps when the voltage drops to try to do the same work. A KW or ZW can deliver more amps than smaller transformers, so that combats the effect of voltage drop.

The two throttles do turn in opposite directions to change the train speed, which takes getting used to. Flip the orange handle to operate the whistle, and press the silver buttons to change each train’s direction. In the event of a derailment or another short, the red light lights up and the circuit breaker cuts off power.

The KW as an accessory transformer

A KW can cost half as much as a ZW even though it delivers 2/3 as much voltage. This makes it a popular choice as an accessory transformer.

The combination of posts C-D gives a very useful 14 volt fixed output. Posts U-D give 20 volts on early models, 19 volts on later models. 19 volts is a good voltage to use on the big Lionel 022 switches. For smaller O-27 switches, I prefer 14 volts.

The KW also offers a 6 volt option with posts U and C, but this output bypasses the circuit breaker. Don’t use this one unless you connect an external breaker, such as a Lionel #92, between the transformer and the accessory.

Beware, though, if you use the throttle to deliver output to accessories. If you never move the carbon rollers, they get brittle over time from heat buildup. Using the throttle distributes the heat a bit when running trains. When using the KW to power accessories, be sure to move the handles from time to time, increasing or decreasing the voltage by a half volt to a volt.

This weakness doesn’t apply to the fixed voltage outputs, so if you’re using fixed voltages, don’t worry about it. But if you want a fine-tuned accessory output, an LW or an RW can be a better choice.

That said, a KW is a better choice as an accessory transformer than a V, Z, or ZW, because it has the three fixed outputs. All of the V, Z, and ZW’s outputs have carbon rollers on them.

But the KW isn’t necessarily the only choice. Here are the pros and cons of various Lionel transformers.

3 thoughts on “The Lionel KW transformer

  • October 2, 2017 at 10:11 pm

    The KW has 190 watts for 2 engines or 95 watts per train. A LW has 125 watt for one engine and two cost less than a KW. LW has a lighted dial that indicates the voltage going to the track. LW puts out 20 vac.

    Two LWs for 2 trains are less crowded for two operators.

    LWs for me


  • November 15, 2017 at 6:45 am

    I have an American Flyer 12B transformer that uses wiper contacts instead of carbon rollers. The voltage output is supposed to be 7-15 volts variable or 15 volts fixed. Mine is 4-6 volts and 6 volts, respectively. Any ideas why the output is so weak?

    • November 16, 2017 at 10:00 pm

      Since it’s outputting about half what you would expect, that makes me wonder if it’s a transformer designed for 220 volts instead of 110. That’s the only thing I can think of, unfortunately.


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