Thread:Steam Locomotives/@comment-31365078-20190411011218/@comment-134861-20190411133739

https://www.thepunctuationguide.com/apostrophe.html

"The general rule is that the possessive of a singular noun is formed by adding an apostrophe and s, whether the singular noun ends in s or not."

Davis is a single person, not a group of Davi, so it is written as Davis's.