According to the grammar book taught in Greek schools, 'Γραμματική Νέας Ελληνικής Γλώσσας Γυμνασίου (2008)', 'την' only keeps the final 'ν' in front of words beginning with a vowel or 'κ, π, τ, γκ, μπ, ντ, τσ, τζ, ξ, ψ'. However, 'τον' (the masculine accusative article) never drops the final 'ν' in writing. So, you should write 'τον διάδρομο'. In spoken discourse the final 'ν' it is usually dropped, unless it is before a word beginning with a vowel or 'κ, π, τ, γκ, μπ, ντ, τσ, τζ, ξ, ψ'. So, most people would say 'το διάδρομο'.