I guess I am missing something here: Seems like the truck could carry around 45,000 lbs. of explosives (ammo?). Granted it was not an obvious military vehicle. And, there is considerable commentary that the PATTERN of the blast and damage doesn't match a truck bomb.
OTOH, while vehicles going over the bridge get screened, bribes are pretty much the norm for Russia.
There was a post yesterday about the bridge bombing that now has over 200 comments. Don’t remember the name, but comment #81 has the best video of the bombing I have seen. It shows three white trucks coming down from the slope of the high bridge. As it reaches the level road suddenly explodes. Some think it was a suicide bomber. There were 2 trucks and 2 cars going the other way. Three people died, probably 2 in one of those 4 other vehicles. The one thinkg I find strange is why there was a big quantity of spray over everything from the right side. Did a bomb hit the water on that side or did the original truck explosion kick up spray from below that got blown over from a strong wind. If the truck was headed toward Russia, then the wind might have been from the south. If headed toward Crimea, then the wind might have been from the north. There were 3 chuncks of 2 land highway taken out. Two were close together and were probably from the truck. The third was some distance away and might have been from some other explosion. That separation shows up in some of the other twitter videos.
The fire on the rail line apparently occurred when the explosion of the truck set a or some oil tankers on fire. The fire was out by mid day. Apparently one of the two rail lines was not badly damaged and they were testing it by running a train with an engine, 14 boxcars, and a tank car over it. They predicted having a passenger train run over it this evening. This bridge project was Putin’s baby. No doubt that is why it was bombed on his birthday. It is the longest bridge in Europe. Four traffic lanes and 2 rail lines. Putin was proud to have finished in 2019 a project that took more than a century to build.