@Angelicsailor It's a 30 years diff yes, the husband is 46 yo. It's spoken off though, practically everyone around them disapproves and they aren't really husband and wife in practice - they don't even share a proper kiss before he dies. Mariko did love/have a crush on her husband, but there was no actual romance - she is a child in his eyes. The ML on the cover is not her husband (that guy is 19 yo, so age gap of 3 years only)
Further
it's not at all like Murder on the Orient Express (by Agatha Christie) actually! I was a bit disappointed by that, but it's an enjoyable story even so. They're only on the train for a few chapters before they reach Paris. By they I mean Mariko and all the people she befriends on the train. The summary is a bit misleading, it only describes the start of Mariko's journey
But her life takes a rapid turn when her husband is discovered dead after the first night on board... And that's only the start of Mariko's great adventure.