Simutrans is free. Some people have tried to sell it, sometimes under a different name, but those are at best scammers. Whether it is safe is a question with a complex answer, but (almost) every user on this forum believes it was worth the risk. You should at least download it through simutrans.com (although that will ultimately lead you to SourceForge) or Steam.
Although Simutrans has been around for many years now, it might not be what your father played. He might have played Transport Tycoon or Transport Tycoon Deluxe, which Simutrans is heavily inspired by. But Simutrans aims to do things better than Transport Tycoon did.
