Not that it matters much but in the videos I was sent the train heading towards Crimea was running at not much more than walking pace. Very little blast/frag in this explosion*, little mid-sized rubble spread and the light debris was taken away by the breeze. The whole scene looked swept clean in the immediate aftermath.
Usually best to ignore apparent blast positions on video (due to frame rate, compression et al) but at the start of the event there is very little damage to superficial / tertiary structure (lampposts etc) or even on things normally cast aside by explosions much smaller than this (eg at least 1x car and 1 x truck that would have normally exited the stage (...pursued by a bear?).
So with the evidence (as of yet) we can probably put aside impact blast, airburst, truck bomb or span demolition charges. Even direct demolition of piers looks doubtful. So what we are left with is a scene that looks like ground heave alone, albeit with a bit of water above it. On one of the videos the seat of the explosion looks to be located adjacent to the northern side of the road deck. It was truly significant charge with deep and heavy debris breaching the surface and making it as far as to land on the collapsing span, followed by the deluge of water. It had enough energy to lift some above-surface structural items with it (inc the reinforced piers) and also caused numerous small self-extinguishing or doused steel fires on the immediate surrounding area.
The effect on the sub-surface structures and underlying bed will have been significant and well-beyond my knowledge (yes, I'm a weapons effect specialist but at this level we would be leaning almost entirely on the scientific staff). Pretty sure their first phrase would be '...a complex interaction that may be difficult to model...'.
The effects on the train are very similar to the scene close to the blast. Little or no effect on the train, no apparent derailing, tipping, decoupling or toppling. Very little medium sized debris made it up there beyond that carried by the water spray. If not for the cargo on the train it would almost be a non-event to the rail element (well, apart from fleshy things not liking blast waves) but the footings would need to looked at with a specialist team. Unfortunately high-speed burning metal meeting big tin-cans of POL is a chemistry event. Given the traverse wind and sequential cooking-off of the tanks spells misery for the the rail bridge structure. The blow-torching of steel both changes its material properties for the worst and makes it sag, stretch, expand at points various. The concrete will also have immediate spalling plus a whole host of micro-fissures that will allow a visually 'ok' bit of concrete fail very quickly due to the maritime environment.
An interesting one for sure and I hope more info is revealed so a more credible effort can be made to understand it than my first attempt above.
(* I acknowledge that there appears to be a signature event in at least one of the blast videos - this may or may not be the case but best not to comment further on such things.)