There were many many conventions for crossing Power/Phone/Telegraph lines accross water near Rail bridges. However, in Trainz, about the only way to do it these days and make it look at all "likely" is to drop a spline Point for every Pier/Abutment (Or in the case of Trestles, every given increment that makes sense), and stick the pole into the bridge Poly's somewhere it makes sense. I usually stick a Locomotive or a few cars on the track to make sure the clearances work out, but I've been pretty happy with the results so far. As you've seen, the common way was side mounted Carrier Arms, but as another user posted, and I've seen examples of elsewhere, it wasn't unheard of to just strap poles to the bridge superstructure itself.
I will say that for the most part Its been pretty rare I've seen poles sunk into the water. Too many things can go wrong with any kind of current. Creeks, Ponds, slow moving Rivers, or dry rivers yea that makes sense, but I've never seen em sunk into even mid-sized rivers or anything that could even hint "Current".
Falcus