To use shaped charges as several people have suggested, they need to be in direct contact with the metal you are are trying to cut. Placing a charge even a couple of inches away will not cut a train rail.
Placing a boat load of explosives under the bridge even if it is just 20 feet between boat and span will loose a great deal of its shock wave effect (Which is what you use to destroy a fixed structure).
A lorry load of explosives would be the same issue, the bast would likely result in a crater roughly the size of the vehicle and would not be likely to create enough of a shock wave that brings down several non adjacent spans.
This could be an inside Rusk escalation with charges attached to the joints of one of the bridge junctions, including an accelerant / vaporiser.
The second option is a very large air delivery munition, by choosing a lower span, there is more opportunity for secondary shockwave reflecting back off the sea. I wonder if the lower height of the piers means. less flex in the sea creating a more rigid structure than targeting a higher level pier.
Is a long time since I did dems but we always used to look a channelling the shockwave.