.

 If the cooled liquid is released in a spill during transport it will warm. As LNG warms the liquid begins to vaporize. 

The gas produced by this warming is typically methane.

If a spill occurs and the vapor does not ignite, it would build to higher concentrations. 

At higher concentrations, vaporized methane will cause an asphyxiation hazard to anyone exposed. 

If a spill or leak followed by a vaporization event were to occur in or near water, then water in contact with the spilled LNG can accelerate the vaporization process and increase the concentration of vapor in the immediate area.

Also, if water is involved, a BLEVE andor an RPT is possible, those are industry euphemisms for explosions.

Of course, in a train wreck, think metal on metal, sparks are likely.  Ignition sources will lead to an explosion and likely fire, which will lead back to the source.

And liquid methane can cause cryogenic burns.

Yeppers, perfectly safe.  What could possibly go wrong?