The fact that the questions that exhibit this pattern keep recurring on this site indicates that it is a pervasive problem with the IELTS that those who write it apparently expect the texts to be interpreted in the way in which they would be interpreted in a fast-moving casual conversation, by people of only moderate attentiveness, rather than in the way in which a lawyer would analyse a legal document, or the way in which a conscientious student is expected to read, say, a mathematics textbook.
In this particular case, the authors of the test expect the candidates to interpret the crucial clause 'if you can make your main journey by rail' as if it were 'only if you can make your main journey by rail'. Now, in casual conversations if is indeed sometimes treated as if it were only if, but everybody who has taken an introductory logic course is likely to be familiar with the difference between the logical relationship expressed by if and by only if, when they are used carefully and precisely. The authors of the test apparently expect the candidates to either not be familiar with that difference or to somehow figure out that they are supposed to deliberately ignore it.
The question also assumes that it is obvious that your main journey is interchangeable with the major part of your journey; that may be debatable.
In a comment, Mr. Ashworth has diagnosed the problem in terms of the authors of the test expecting the answers to take into account the pragmatics, without making that expectation explicit in the instructions. Pragmatic considerations are, however, supposed to be sensitive to the context, and in this case 'if you can make your main journey by rail' occurs in the context of some formal rules about the validity of rail-and-bus tickets, and not in the context of a casual conversation. The considerations of pragmatics would tell one that such a quasi-legal context calls for careful scrutiny of the text and attention to such details as whether it uses if or only if. The OP's reasoning is thus correct even if one takes pragmatics into consideration.
Common sense suggests that you wouldn't buy a ticket at a railway station unless you were intending to make the greater part of the journey by train, the train station might be the only place to sell such tickets (we don't know), whether the train portion is major or minor. – Ivan Balashov Nov 14 '20 at 13:41