On many Marx trains, the driving gear extends to the full width of the drive wheel, making that wheel effectively wider than the other one. This causes interoperability issues with other manufacturers, especially on switches and crossings.
A very common question, therefore, is which Marx trains lack the dreaded fat wheel.
The easiest answer is the Marx 666 and the Marx 333 and their plastic-bodied counterparts, the Marx 1666 and Marx 1829. These two steam engines never have the fat wheel, so they work well on other makes of track. The large plastic Marx E7 diesels fall into the same category.
With other engines, you unfortunately have to look. There were some Marx 590-series and 400 locomotives, for example, that had the newer, double-reduction motor in them. Also, if someone sent a Marx locomotive in to the factory for repair after about 1952, Marx may very well have put a double-reduction motor in it.
There are two places to look, depending on what pictures are available if you’re buying online.
Look between the wheels
First, look at the gears between the drive wheels. If there’s one large gear visible between the two wheels, it’s a single-reduction motor. If there are two medium-sized gears visible between the two wheels, it’s a double reduction motor.
“Double reduction” is a good term to watch for. A train seller who knows his or her stuff knows the difference between the two and will probably mention it in the listing. So sometimes you can get lucky by just doing an Ebay search for “Marx double reduction” to find engines that can work with Lionel, or a loose motor that you can swap in to a locomotive you already own.
If you can get a view of the underside of the motor, you can tell from looking at the drive wheels themselves. A fat-wheel engine has teeth right up to the edge of the wheel. On a double-reduction motor, the gear on the drive wheels might not even be visible from the underside because it’s much smaller.
There are other advantages to double-reduction motors, besides the ability to navigate Lionel switches. The double reduction motor runs more slowly than the single-reduction variety, which runs at nearly supersonic speeds. Also, the double-reduction motor has greater pulling power. For these reasons, it’s not uncommon for a Marx engine with a double-reduction motor to sell for about $10 more than a comparable engine with the single-reduction motor in it. The premium is worth it.
Also keep in mind that it’s very easy to swap a double-reduction motor from a beat-up 400 or 590 into another locomotive, so if you find a beat-up engine with that motor in it at a good price, it’s worth buying. I always look for beat-up Marx engines at train shows to get a good look at the motor. The double-reduction motors are desirable but they aren’t particularly rare, so you’ll find them more often than you might think.