Do we know it was Ukraine that did this?
Looking at the missing bridge spans, this looks nothing like a missile strike.
This looks like a commercial demolition job. The type where they take their time and drill holes in the concrete for precisely laid explosives. Not the sort of thing that is done on the stealth while the guards are looking the other way.
we have no real information at this point, so everything is pure speculation but we have the car bridge damaged and a train on the rail bridge affected in a seemingly significant way.
if it was a missile/artillery strike at long range with much less accuracy then we might expect both the rail and road to get hit.
if it was special forces planting explosive charges then you would expect the rail line to be higher priority if you want to stop military supplies. but the rail line is for now intact.
assuming a UKR demo team went in at night, having them waiting around for a fuel train to cross at the right time seems risky but if the target is the rail-line then maybe worth it. theoretically you would bring enough explosives to 100% destroy the rail line and maybe use anything left over to take out the road. but the road is down and the rail span is still up. relying on the fuel train fire to finish the job seems chancy. special forces carrying that much explosives would seem like a lot of extra weight to be hauling underwater. the other option is they took a car and loaded it with explosives and timed it for when the train was nearby. but again the train line seems like the more pressing target.
it would depend on all those above and underwater patrols the rus supposedly had guarding the bridge.
the damage appears to run across both bridges. so that generally rules out an aircraft attack, since they would want to fly a path along the bridge rather than perpendicular.