Other than Marklin, what HO locos require a 3rd rail?
I am not aware of any commercial products other than Marklin which use center third rail (or studs) and AC motors. However, back in the dim, dark days of my childhood, there were HO locomotives which drew power from third rails mounted to the sides of the tracks. The same system was common in O scale, and lasted until much more recent times. (John Armstrong's Canandaigua Southern ran with third rail until his death in 2002. There may still be some survivors.)
Chuck (Modeling Central Japan in September, 1964 - 2-rail analog DC)
IIRC, Marklin uses AC with their third rail. Trix is their two rail DC brand.
Many years ago just about everything was three rails, but in time HO and the smaller than O scales went to two rails. Many people also did outside third rail in HO as well. This probably had a lot to do with manufacturing techniques and materials available at the time. A lot of the track was built with steel ties, as plastics were not widely employed yet, nor where the variety of types we now have available. So they would have kept that style for HO. Easier to insulate everything that way.
LION has third rail for the subway trains of him:
But alas, they are dummies, just like the rest of his LIONS.
Ach, but I betcha I could make better third rails today, but still, just for looks.
ROAR
The Route of the Broadway Lion The Largest Subway Layout in North Dakota.
Here there be cats. LIONS with CAMERAS