Jonathan Mauchline of Levin asks :-
During a recent drive we came to a railway crossing. The barrier arms were down but no train was coming. I would like to know what makes the barrier arms go down when there is no train coming.
Roy Percival, a railways signal engineer with Ontrack, responded.
Level crossing alarms, that is, flashing lights and bells and automatic barriers, are designed so that the alarms start at least 20 seconds before the fastest train reaches a level crossing. The level crossing alarm control system sends an electrical signal along the rails so that when an approaching train passes a certain spot on the track the alarms start.
Railway level crossing alarm systems are designed to be "fail safe". This means that if anything goes wrong with the alarms the alarms are designed to start to tell people that a train might be approaching. The flashing lights and bells and barriers operate off batteries when the main power fails. If the battery power fails then the barriers fall down under gravity as part of the "fail safe" system.
When a railway crossing is near to a station platform such as at Levin, we do not want the alarms to operate when a train is stopped in the station. This means that there is usually a railway signal that shows red until the train is ready to leave. In this case the alarms at the level crossing only start when the railway signal is put to green for the train to carry on its way.
In answer to your question, there are three main reasons why barrier arms might be down when there appears to be no train coming: 1. There has been a main power failure at the crossing. The electric signals in the rails that tell the alarm system that a train is coming often need the main power supply to work correctly. 2. There is a railway station near the crossing and the railway signal has been put to green before the train is ready to leave. Although the Train Controller in Wellington tries not to put signals to green before a train is ready to leave, quite often the train may be delayed in starting because of late passengers or extra luggage. 3. There has been some kind of damage to the alarm system such as a broken wire.