Correct me if I'm wrong, but I think that мост is "irregular" in that the locative case and the prepositional case are not the same. The prepositional is the same, i.e. мосте, but the locative is мосту. That means that "we are on the bridge" (using the locative) is "Мы на мосту", but for example "I read about the bridge" would be "Я читаю о мосте". There are a few other Russian words that work the same way, for example "Crimea" -> Крым - в Крыму, о Крыме.
Hope this clarifies it
Thank you for the answer! However, your answer raised a question for me which I hope someone could clarify.
I have learned that Russian has 6 noun cases: Nominative, Accusative, Genitive, Prepositional, Dative and Instrumental. I have never heard of a "Locative" case. What do you mean by this Locative case?
Russian has the vestiges of a separate locative case. See https://en.m.wikipedia.org/wiki/Locative_case#Russian Other Slavic languages still have locative as a real case