The span is the most attractive feature of the bridge - that's the one part that should remain if possible.
I suspect their main concern is that a rotten wooden plank might fall onto the track, which is fine except properly maintaining the flooring wasn't a problem during the previous 100 years or so. (Exactly how old is this bridge?).

If the span has to go, surely it would be much cheaper to buy and install an off the shelf structure with circular ramps at either end?