When a guy lays his hard penis on his belly and his girl slides her dripping wet pussy on top and glides along it like the Monorail at Disneyland.

MONO = ONE
RAIL = RAIL

One-rail mass-transit, you can find one in Sydney.

Mono means one and rail means rail.

Sydney's monorail exemplifies excellent town planning, just like the popsicle stick skyscraper, the 50-foot magnifying glass and that escalator to nowhere.

for a girl to grind a guy who is laying down, esp. with his penis between her labia minora

When you pin someone down, then run your Arse Crack along their nose.

When an individual is held down against their will whilst an agressor positions his buttocks either side of the poor soul's nose. The "crack" is then rubbed up and down the face in a straight line, mimicing the movement of a monorail.

When two guys put the tips of their erect dicks together and one guy strokes both dicks like it is one large dick.