Mashinky is (way) more casual than OpenTTD but it's a really nice game made by a solo dev.

The vibe is similar though and you can even take a break and ride your trains. I strongly recommend giving it a try.

https://mashinky.com/

Transport Fever is great. It is beautiful and has an engaging single player mode. Now there is also Transport Fever 2, but I would start with 1, because it is cheaper and still totally enjoyable.