Asbestos fibres are dangerous when inhaled which is why it was phased out as a fire retardant insulation material and ingredient in other building products. In fact, where asbestos panels remain in buildings it is perfectly safe provided that the panels are not damaged or disturbed. It's only when you start bashing it about that it is in danger of releasing asbestos fibres.
As weird as it may seem, submerging complete rail vehicles is a good way of safely disposing of asbestos contaminated stock. The dangerous asbestos is not disturbed and being submerged in a flooded quarry means that the fibres cannot be released into the atmosphere. Even if asbestos somehow found it's way into the drinking water supply there is no evidence that it is dangerous if ingested.
It certainly looks wrong to dispose of asbestos contaminated vehicles in this way, but it actually makes a lot of sense and probably isn't as dangerous as it appears.
O L Leigh