Monorail, meaning 'single-rail', is based on an existing technology, where a train is travelling on a single beam, rather than two separate tracks. Several countries in the world use monorail for daily train service.
In game first Monorail train ('X2001' or Wizzowow Z99) is available in 1999.
There are the two Monorail trains available in Temperate, Sub-Arctic and Tropical climates:
And one train in Toyland landscape:
None of the Monorail vehicles in OpenTTD has a real-life equivalent, they are purely fictitious.