A dedicated communication device used in manual block signalling to coordinate train movements between adjacent control points. The instrument typically features bells, plungers, and visual indicators that show whether the track section ahead is clear or occupied. Operators use a strict bell code on the instrument to request and grant permission for a train to enter the block.
Heard on the job
I rang the block instrument three times to offer the manifest, but tower 4 hasn't acknowledged.
Insider tip
Though largely replaced by centralized traffic control, these instruments remain the backbone of absolute block working on older or heritage lines.