While many rail museums have this problem, there is nothing about being a "professional curator" (which I assume means they are paid instead of a volunteer) that prevents someone from behaving in the way described. In fact, some curators get this exact same mentality because they feel "better" than the volunteers as they are paid, thus believing the museum belongs to them more than some volunteer.

In addition, many railroad museums are small operations and simply can't afford the cost of a professional curator.

Finally, rail museums are a much different animal that most art and history museums. Especially those that operate trains instead of just displaying them. The need for volunteers is much greater and a decision to restore "Locomotive X" might be the best for the museum but its meaningless if nobody is interested in working on that particular piece.