Copyright protects “original works of authorship” that are fixed in a tangible form of expression. If the person is objecting to the name of the train, then I do not see the legal problem presented. I recommend that you show the letter to an attorney for specific advice. I would however, expect that the person objecting is claiming copyright rights to the photo itself. Titles, names, short phrases, and slogans; familiar symbols or designs; mere variations of typographic ornamentation, lettering, or coloring; mere listings of ingredients or contents are not eligible for federal copyright registration. The name of the train may fall into this category. You should check with the U.S. Library of Congress as to whether or not the work is registered as a copyright or check with the owner claiming rights. There are further articles regarding copyright upon my website that you may find of use. I do not believe that you are using the copyright in a fair use manner since you are deriving profit from the sale upon the internet. You may inquire with the registered owner or owner claiming copyright about a licensing agreement or royalties agreement. The owner may be agreeable to receiving a portion of the profit. An attorney can prepare a simple licensing agreement for this purpose.
There can be many rights involved in selling a photograph, including the photographer's/author's rights in the photo itself, and the rights of the people and entities pictured in the photo. It sounds like you only have rights to the copy of the photo itself and you have not given any consideration to any other rights.
If the name of the train is trademarked, fair use allows you to use the name to nominatively refer to it, if you give proper attribution to the tradmark owner and don't use the name any more than necessary to identify it..
You have gotten some good advice, you should take the photo and the letter threatening legal action to a qualfiied lawyer who can advise you regarding how to treat this photo.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
The below does NOT constitute legal advice, does not form an attorney-client relationship, and should not be relied upon to take or refrain from taking any action.
As with any legal dispute, seeking the advice of an attorney is always best. In your case the advice would likely cost far more than you'll make if you sell the photographic negative. I'm not your attorney and so the following is simply how I would personally respond if faced with the situation you describe: I'd ignore the complainer and auction off my negative. If that ticks off the complainer, so be it. He'll have a heck of a time finding a reasonable lawyer to file suit against you (which is his only recourse).
Based on the information you've provided, the complainer owns no copyright interest in the negative. (Someone does, however, so don't make copies from the negative and sell the copies without clearing the rights first.)
As far as trademark rights in the name of the train that's shown in the negative, those rights (if there are any -- note that trademark rights do not automatically arise simply because someone named something) do not extend so far as to prevent the sale of photographs that contain the trademark.
My take on the matter is that you're not making a "fair use" of the trademark -- you are, more fundamentally, NOT making use of the trademark at all. You're offering to sell a photographic negative that, incidentally, shows a trademark. Happens all the time. But that offer to sell the negative is not a "trademark use" of the trademarks that happen to appear in the negative. The "trademark use doctrine" precludes a finding of infringement in situations where the mark is not being used "as a mark."
There are always more facts in play than are first identified -- so the safe bet is to discuss this matter with a trademark attorney. But safety has a price. Good luck.