Which of the original Monopoly railroads is not based on a real railroad?

The Short Line Railroad.

The Monopoly game's Short Line isn't named for a real-life railroad. The game's inventor is reputed to have had a small interurban tram line, the Shore Fast Line, in mind when he named the game property. The term short line applies to any independent railroad with tracks covering a short distance or small region.

https://www.thesprucecrafts.com/monopoly-railroads-2382515

In the U.S. version of Monopoly, the railroads were:
* Pennsylvania
* B & O
* Reading
* Short Line

Those more familiar with the U.K. version will remember these railway stations:
* Kings Cross
* Marylebone
* Fenchurch Street
* Liverpool Street
All these stations were real termini for the LNER (London North Eastern Railway.)