But aren't there additional systems that can detect that a block is obstructed (e.g. also by left behind carriages that got disconnected) like the electrical track circuit (Gleisstromkreis)?
Or wouldn't that have helped because we're back again to the issue that with the downgrade from LZB to PZB we lost the virtual blocks, and both trains are now occupying the same, longer, non-virtual block?
Which would mean that this block would have been indicated to the controllers as not clear - but that is exactly what they would expect to see, because they're currently deciding what instructions to give to the AVE, which they know is stopped in that block. But they have no indication that there is a second train (Iryo) in front, occupying the same block.
Hence, I assume, the protocol to resume operations with drive by sight after a LZB failure (which seems to have been ignored in this case). But why not also have the protocol demand that operations are resumed front-to-back in that situation? I suspect that "front-to-back" is not nearly as unambiguous a concept in reality as it may seem in my layman's head.