A fire certainly will limit supplies over the bridge. Hopefully for a long time.
Kerch Bridge is a great psychological hit for Ukraine. If a lot of stuff was staged for movement in Krasnador, it will have to go further up through Rostov-on-Don now. A high percentage hit to material flow. But not crucial. Even if the bridge is repaired, further movement will be slowed due to tighter security checks on everything. One shot, continuing effect.
Not to mention the impact on Russian tourists! You know, the kind of people who go home and talk about how horrible their Crimean vacation was because Putin can’t keep things under control.
There are more photos and videos in the link.
So the rail bridge is on fire and the road bridge is buckled into the sea?
How did this happen? 
Rail sections would have to be heavier construction at minimum. Maybe something on the roadway blew up, destroying those sections. But only led to fire on the railway section? Some opportune timing of vehicle bomb detonating while train was parallel? Or that train train crew is just F it, damn the flames full speed ahead. They can always disconnect and get there on time, if a little light. Sucks for caboose crew, if they still have them.
Looks like a tanker train.
Happy Birthday, Vlad.
My guess is infiltration team with a drone. Don’t need a big warhead if your target is fuel tanker train cars.
That tanker train didn’t cause the complete collapse of multiple spans of the far side of the roadway bridge. This was sabotage or missile strikes. Both the vehicle and train spans are currently unusable. Crimea is now only connected to the mainland via Ukrainian (if not currently Ukrainian-controlled) territory. This make me wonder if Zaporizhzhia “feint” isn’t a feint. If the Ukrainians can get within shelling distance of Melitopol, then Crimea is cut off - as would be Kherson. It would fit with Ukraine’s general tactical/strategic brilliance in this war so far. Why fight them there? Cut off their supplies and wait for them to run or surrender.
Wow, in the latest images I’ve seen, it appears that both road spans are now down and the train span may be buckling from the heat. Doesn’t look like there’s much chance of putting out those fuel cars anytime soon, either.
My guess is in agreement with @Zakalwe . Something explosive, either planted explosives or a missile strike took out the road spans, and that set off the fuel cars. If they knew the train would be there at that moment and it wasn’t just luck, give the person in charge of this event a medal.
You can hook a bomb to a detonator that is triggered when GPS coordinates are reached.
A Zaporizhzhia offensive wouldn’t even have to get to Melitopol. Unless I’m missing something, the east-west rail link is a single line running through Tokmak that’s only 30km behind the current front line. That puts any rail bridges on that stretch easily in HIMARS range. Actually, in range of several tube artillery types as well. Could be a very bad week for Russian logistics.
Unfortunately Ukraine does not have as much forward artillery as would be optimal for this. May be some other options now or in near future. But it has been a problem for their offensive operations that they do not have enough artillery.
https://dlca.logcluster.org/display/public/DLCA/2.4+Ukraine+Railway+Assessment
It’s not like you need a thousand guns to take out a box girder rail bridge. A couple Caesars or PzH2000 should be able to bring a bridge down. Relatively easy to repair a box girder bridge, but you can just hit the repaired section again every time they fix it. Cutting Crimea and Kherson completely off from rail transport would be well worth dedicating an artillery unit to full time rail interdiction through Tokmak.
That’s assuming, of course, that tomorrow the Ukrainian army doesn’t launch a massive offensive and just take Tokmak outright.
Two spots where the single rail line running between through Tokmak looks pretty easy to wreck:
A bit east of Tokmak, 34km from current front:
Southwest of Tokmak, 50km from current front:
Mind you, this could be a bit moot if it turns out the rail span on the Kerch Bridge isn’t actually compromised once they get the fire out. I’m not seeing any sagging of the rail span in the video, just the walkway along the side of the span letting go, but I can’t imagine that that’s a structural part of the bridge.
If you are in range of Tokmak, then you are likely in range of a lot of Russian artillery and more. Of course you can try for the hit. But it is a tuff job. They shoot back.
Oof. Just looked more closely at that rail map, and the situation without the Kerch Bridge is worse than I realized. The rail network in the southwest only links up to the network in the southeast through Donetsk city, and the rail line is within a few km of the front line just to the southwest of Donetsk near Vuhledar. Close enough that you could use drones to spot trains and target trains using M777.
It’s only 30km from the front. Ukraine has kept the Dnipro bridges in Kherson out of commission from further away than that.
It is very close there. Donetsk is shelled regularly. But not very heavily. I would not be surprised at all if they could get spotters to help. Again a tough job. But it is not impossible to take out rail lines in this situation. Fairly easy to repair, unless you can actually destroy a rail bridge. That is a prize.
Russia says it was a truck bomb. Which actually seems plausible, but since they never tell the truth now I don’t know what to think. 
Boom
Big boom
That second one makes it look like the explosion originates below the road span? A ship bomb?
Any which way, that’s a big fucking fireball.
Here’s some footage of the remnants in daylight;