This is the 11629th most frequent Russian word.
железнодорожным
"Relating to railroads or railway."
'железнодорожным' is used as an adjective describing 'транспорте' (transport), indicating its relation to railways.
Я путешествовал на железнодорожным транспорте.
I traveled by railway transport.
Here, 'железнодорожным' describes 'мостом' (bridge), specifying that the bridge is related to railways.
Железнодорожным мостом пользуются ежедневно тысячи людей.
Thousands of people use the railway bridge daily.
In this sentence, 'железнодорожным' modifies 'сообщением' (communication), identifying it as pertaining to railways.