Thomas Neazor-Grant, of Greytown, asks :-

My dad could not go to work because a cow derailed the train he was going on. How can a 270kg cow derail a train?

John Campbell, a physicist at the University of Canterbury, responded.

Quite easily. However it isn't the collision that is crucial because a very heavy railway engine (typically a hundred tonnes) would hardly notice a collision with a relatively light mass (a quarter of a tonne cow). The train's speed would only slow down by 1 part in 400. It is what happens to the wheels that is important.

Trains are very effective and efficient at moving heavy freight because the tracks must be flat, and not very steep because otherwise the wheels could slip. Both track and wheels must be hard-wearing so are traditionally made out of steel. The steel wheels are prevented from slipping sideways off the smooth tracks by the wheels being tapered inwards and flanges on the inside of each wheel.

Each group of wheels are on a pivoted bogie to allow a long, stiff, engine, carriage or wagon to move round a curve in the railway line.

A train, carriage or wagon can derail if a vertical gap opens up more than the height of the wheel flange, which is about 3cm. Hence it is essential that the track is well maintained. e.g. a small washout could allow the track to subside, the wheels of a bogie loose contact with the track, and rotate slightly so it doesn't come back together with the wheel flanges in the right position. Once one bogie goes off the tracks others bogies may be lifted enough and also derail.

Another possibility is a wheel becoming cracked and collapsing. Old timers, or viewers of films of the old west, may recall an engineer at a station tapping each wheel with a hammer. (A cracked wheel wont ring like a bell.)

Old trains had a 'cow catcher' at the front of the engine so a large animal would be thrown aside. The object was to prevent bits of the animal going under the train where the solid bits and/or the fluid bits could cause the the wheels to rise and 'derail'.

These days stock is much better fenced off from railway tracks so such collisions are rare, but obviously still happen occasionally as in your Dad's case.

High speed trains are only allowed on well-fenced tracks. Magnetic levitation trains are an interesting case whereby it would be desirable to have a magnetic field configuration that would centre the train on the 'track'.