Railway track explosives are small explosives which need to be strapped on the railway line for use. When the train wheel crushes it while passing over the explosive will explode with a loud report which is a signal to train drivers to stop the train.

Use of railway track explosives (source Wikipedia)

• a warning, caution or stop signal in dense fog, when signals are difficult to see;
• a warning of a train stopped on the line ahead by an incident or accident — the train’s personnel are usually responsible for placing the detonators;
• a warning of ongoing engineering works ahead; or,
• when a signaller or other railway employee requires to stop approaching trains in an emergency.

Entries of Railway Track Explosives in IMDG Code