Translation:My brother is also traveling on this train, but in a different car.
Similar to other comments, I said "is going" instead of "goes", and I said "on this train" instead of "in this train". In both cases, their English is less natural than mine, IMHO. Frustrating. But, as I have to keep reminding myself, it's not an actual person grading my response, it's software trying to do its best to take into account every possibility, and that's a very tall order.
It’s because Duolingo expects you to use American English, not British English. Confusingly however, many British English words such as shop, film and trousers are acceptable. I assume that it’s because more non-native speakers learn the British version and this has crept in. As a native BrE speaker, I just feel very grateful to have Duolingo and move on.
In Britain, “to go by train” is standard English usage. Indeed, it is perfectly normal “to go by bus”, “to go by taxi”, “to go by car” or by any vehicle. Although this course is supposed to be in AmE, some of the Russian speakers who voluntarily assembled it were more familiar with BrE usage.