A lot of people think the only trains in Venice are in the Santa Lucia Station, but there is one pair of tracks that cross the Grand Canal in the direction of Tronchetto. I've examined it from aerial photos and they're clearly not there for turnarounds, so my guess is they're used for freight loading, and in this case, to get a train off the track so another can come in it's place.
Maybe they use that area for servicing that takes too long to be performed in the terminal.
I've been to Venice a number of times, but this was the first time I'd ever seen a train cross the Canalazzo. It was quite a surprise.