H2O

The ratio of hydrogen atoms to oxygen in water (H2O) is 2:1, which means for every one oxygen atom, there would be two hydrogen atoms as well.

Yard - Marshalling yard with artificial mound or hump over which cars are propelled and gravitate to correct siding and position in the yard.
Hydrocal - A US Gypsum product used in model railroading for the base of hard shell scenery.
H2O - ...