For NYC diesel trains, you're talking about stretches like the one from Croton-Harmon to Poughkeepsie (by car, it's about 40 miles). I'm not sure whether this counts as "short."
https://www.reddit.com/r/nycrail/comments/l57nw2/map_of_the_...
(LIRR Montauk branch is longer, but I'm less familiar with it.)