3 Rail Track, an eBay guide on Märklin HO Trains (Marklin HO Trains)

There are many brands of HO scale model trains, and most brands are usually compatible with each other.  However, the brand of Märklin, which comes out of Germany, is fairly unique and is generally NOT compatible with many other brands of HO Trains.  Here is why:

2 Rail versus 3 Rail:

Most brands of HO scale model trains use 2 Rail track.  There are 2 rails, one with positive (++) DC power and the other rail with negative (--) DC power.  Most American HO trains are 2 Rail DC; the famous Fleischmann HO trains from Germany are also 2 Rail DC.

However, Märklin HO is a 3 Rail AC system.  Both rails (outer rails) are electrically connected to each other and are “ground”.  The middle rail (aka the third rail) is isolated from the outer rails, and it supplies the “AC power”.  In the more modern types of Marklin HO track “M”, “K”, and “C” track, the third rail is very hard to see.   It is not a solid third rail; rather it is a series of studs that stick up out of the “sleepers”.  (Look at the picture below, it is very hard to see the third rail studs sticking out of the black sleepers.)  A slider on the bottom of the locomotive runs along these studs to pick up the power. These are a commonly replaced part due to the extreme wear and tear (different locomotives use different sliders; see link below). 

Märklin HO Sliders

The advantages of the Märklin 3 Rail system are:

• A key advantage for 3 Rail track is reverse loops, where a train enters a loop through a turnout and then exits through the same turnout in order to change the train's direction. With two-rail track, when the track reverses on itself, this causes a short circuit. With three-rail track, because the center rail remains constant and the outer rails are electrically identical, this causes no problems.
• Another advantage for 3Rail track is the additional surface area for contact between the track and the locomotive for power transfer.  Bad electrical contact between the train and the track can become an issue especially if the track has become “dirty”.  3 Rail track is more forgiving than 2 Rail track.
• The third rail has also been used to automate and animate layouts. An accessory, such as a railway signal, can be wired to a section of track that has had one of its outer rails insulated, either at the factory or by a hobbyist. A passing train then activates the insulated rail, completing the circuit and causing the accessory to operate.
• Insulated rails (or rail sections) can also be used to control turnouts, causing the turnout to switch to the position needed by an oncoming train.

Because of this feature, railroad cars intended for three-rail operation will not work on two-rail track unless their wheels are first insulated from each other. Cars intended for two-rail track will operate on three-rail track, but they will not activate controls wired to an insulated rail. Conversion of three-rail cars for two-rail operation, or vice-versa, is thus a common practice among hobbyists. It requires either replacing the bogies (wheel assemblies) on the car, or replacing metal axles with axles made of a non-conductive material.

In conclusion:

• Locomotives designed for 3 Rail such as Märklin will not work on a train layout using 2 Rail track, and vice-versa. 
• Cars (rolling stock) designed for 3 Rail such as Märklin will not work on a train layout using 2 Rail track.
• Cars designed for 2 Rail will work on a Märklin 3 Rail layout, but some of the clever automated control systems might not work correctly.

Timothy and Andrew Eckert  
Eckert’s Marklin Trains and Parts

Eckerts ebay store link