German - "Mit der Bahn fahren wir." - I have a further question about this ...?

Mit is followed by a DATIVE, so that is dem, der, dem, according the the gender - mas. fem. or neut. It is "die Bahn", so becomes "der" after the preposition mit. Get it? mit + den is a plural where the plural word "die" for "the" changes to den. Check a grammar book too!