Anyway, you should post the whole list of GRFs that you use, as it is nearly impossible to identify the problem otherwise. A saved game that exhibits the problem could also be helpful.
In the forum's interface for writing a reply, notice that there are two tabs below the text box. Select the tab that says 'Attachments', and click the button in that tab. In a pop-up window, select the file you have created previously.
That's it! Once you click 'Submit', the file will be attached to your new post, and people will be able to download it.
P.S. Oops, I didn't notice that this is a question about the original Transport Tycoon, not OpenTTD. I hope the interface of that game is similar enough so you can find your way around it.
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