The following is hypothetical, since we do not have a proper safety report yet.
It sounds like the issue happened, because there was an error in the line-based train control system (LZB), which led to falling back to the point-based train control system (ASFA). However since there are more virtual blocks in LZB, than physical blocks in ASFA, this means that suddenly more than one train is in one block, which violates the most basic assumption of train control. That requires the trains to not rely on the train control at all, even if it says that the block is clear, but instead are operated manually.
I do not know what the proper procedures for that are in Spain, but in e.g. Germany that seems to require driving by sight. However that is also only after a number of accidents due this situation. So the following seem to be similar accidents from Germany:
- https://de.wikipedia.org/wiki/Eisenbahnunfall_von_Meerbusch-...
- On April 9, 2002, a near-collision occurred near Fallersleben. After a computer at the LZB control center in Fallersleben crashed, two trains on each of the two main tracks came to a stop. When the computer was restarted, the rear train in each pair was signaled a speed of 160 km/h, while the front train in each pair was signaled 0 km/h. One of the two train operators following saw the train stopped in front of him; the other, as a precaution, contacted the operations center, which warned him not to proceed.
It sounds like the issue happened, because there was an error in the line-based train control system (LZB), which led to falling back to the point-based train control system (ASFA). However since there are more virtual blocks in LZB, than physical blocks in ASFA, this means that suddenly more than one train is in one block, which violates the most basic assumption of train control. That requires the trains to not rely on the train control at all, even if it says that the block is clear, but instead are operated manually.
I do not know what the proper procedures for that are in Spain, but in e.g. Germany that seems to require driving by sight. However that is also only after a number of accidents due this situation. So the following seem to be similar accidents from Germany:
- https://de.wikipedia.org/wiki/Eisenbahnunfall_von_Meerbusch-...
- On April 9, 2002, a near-collision occurred near Fallersleben. After a computer at the LZB control center in Fallersleben crashed, two trains on each of the two main tracks came to a stop. When the computer was restarted, the rear train in each pair was signaled a speed of 160 km/h, while the front train in each pair was signaled 0 km/h. One of the two train operators following saw the train stopped in front of him; the other, as a precaution, contacted the operations center, which warned him not to proceed.