Night rain vs Night train, gemination?

In my whole life, I've almost never heard the final /t/ in night and the initial /t/ in train articulated the same way.

Short answer: dialectal variation (and I think it's a poor choice for an example of gemination)

The Wikipedia article you cite qualifies this example as a minimal pair "for most accents" (which I doubt, but that's Wikipedia for you). I also notice that, in that section, it's the only example without an IPA transcription (further raising my suspicion).

You'd need a phonologist and a fluoroscope to distinguish all the variation in "night train"

/ˈnaɪt treɪn/ /ˈnaɪt̚ treɪn/ /ˈnaɪʔ ʧreɪn/ /ˈnaɪt̚ t͡ʃreɪn/ /ˈnaɪʔ t͡ʃreɪn/ /ˈnaɪt͡ʃreɪn/

and "night rain" among English speakers.

/ˈnaɪt reɪn/ /ˈnaɪt̚ʔ reɪn/ /ˈnaɪʔ reɪn/ /ˈnaɪɾ reɪn/

Some of these might still make a minimal contrast pair, but with t-glottalization, pre-glottalization, an unreleased stop, or even a flap potentially replacing one half of the twin pair, it would not necessarily be gemination.

You're not the first person to question that particular example. (see also https://en.wikipedia.org/wiki/Talk:Gemination#.22night_train.22_versus_.22night_rain.22 )