A retro-reflective Infra Red sensor on the tunnel wall is used to detect something – our local instrumentation expert opined that such a sensor cannot detect smoke level accurately, in any case the performance would degrade as soon as dust would accumulate on the lens of the unit. So the sensor could be triggered by the motion of the pantograph of the bankers. Although many trains are worked with diesel locos through the tunnel 25 C, the bankers are always electric locos – hence the system would work satisfactorily.