TODS-E205
service_id in vehicle_assignments.txt is required when the same block_id is used by more than one service. Without it, the assignment cannot be matched to a single block.
Interpretation: per-row reading: fires only for rows whose block_id is ambiguous, not for every row once any block is shared (spec-questions #8).
Spec reference: https://tods-transit.org/spec/#vehicle_assignmentstxt