Who first came up with the light clock on a train thought experiment.
It's in Einstein's original 1905 paper...
On the Electrodynamics of Moving Bodies. Annalen der Physik 17 (1905): 891-921.
I think doing thought experiments in closed moving boxes dates back to Newton or further - with the kid of box being updated for different transport modes as technology changes.
Einstein initially works it out using light propagating in the x direction, then considers the analagous situation with propagation in the z and y directions. The freshman-physics simplification of a light-ray in a box follows naturally.
Thanks but my question was who first came up with the particular light clock on a train thought experiment.
Then I'm afraid you are going to have to define your terms.
The 1905 paper has a light-clock in it (a beam of light traversing a perpendicular path in the rest-frame of the source) - Einstein just does not use those words.
Are you asking who first thought to reword the description Einstein gave in the 1905 paper explicitly in terms of the "light clock" and "train" imagery - that's Einstein again ... probably first used in a lecture, but the imagery appears in his 1916 book Relativity: The Special and the General Theory.
What are you hoping to learn?
I'm just surprised such a widely used thought experiment can't be attributed definitively to an author.
1. There are quite a lot of famous works that cannot be attributed definitively to an author.
2. this one can be definitively attributed to an author: Albert Einstein - in two forms.
This question was asked before on this thread, the earliest example anyone knew of was the 1909 book The Principle of Relativity, and Non-Newtonian Mechanics by Gilbert Newton Lewis and Richard Chace Tolman, see p. 714 online here.
Thanks JesseM, much appreciated.
Separate names with a comma.