That's a surprisingly difficult question to answer definitively. It is by no means definitively "Yes".
ICRC: Rule 7. The Principle of Distinction between Civilian Objects and Military Objectives
You end up needing to balance the military necessity against the presumption that 'civilian objects' should not be attacked.
Targeting the wagon carrying military materiel is likely OK.
Targeting the (civilian) locomotive when it is pulling the (military) wagon likely is probably OK
Targeting the (civilian) locomotive when not pulling a military load is probably not OK
Targeting the bridge used predominantly for civil purposes when no military load is being carried across it is likely not OK.
It is not an easy call, and it is possible to argue that military exigency applies to destroying the Kerch bridge; but there is an arguable case that doing so could, possibly, constitute a war crime. Do the military reasons for destroying it outweigh the civil disruption it causes? If the civil population of Crimea are dependent on humanitarian supplies carried across the bridge, destroying it might be regarded in a poor light.
From a military point of view, the Kerch bridge is an obvious fat target - but because it is not a purely military structure, the justification for attacking it needs to be well thought out and needs to be able stand up in court.