1800's?
Coasters were already fairly well known in the 1800's, and popping up all over the place in some form or another. Definitely by the second half of the century. America got it's first switchback railway in 1827, Western Europe built similar rides before that. The first looping coaster was built in France in 1846. Coney Island existed in the 1800's, opened their Switchback Railway in 1884 and in 1901 opened it's first looping coaster. Blackpool Pleasure Beach opened in 1897, and was inspired by a visit to Coney Island to expand in the 1900's, adding a coaster in 1907.
I don't think there'd need too be much explaining. So we'll have to go further back to the 1700's I think... This would avoid the low hanging 'like a locomotive / train' fruit too. Although I'd probably still just say "A cart running along 2 wooden rails that form several hills, so the cart rides up and down these hills."
P.S: This article is very old, but quite detailed... Not 100% sure of it's entire accuracy, but worth a read.
https://www.washingtonpost.com/arch...coaster/4490a0f9-6a82-451d-86b7-f36a7bc0fbbf/