Train speed
Why are we unable to guess the speed of a train when we stand on the track facing the oncoming train?
SURESH MOHAN
Madurai, Tamil Nadu
When we see a moving train side ways, we guess its speed by the swiftness with which our eyes move along the object by tagging our sight to a point of reference (mostly the engine).
If we see the moving train not necessarily fully sideways but at an angle (say, while approaching), still we can guess its speed by the change in its depth from us because of the binocular vision we have which manifests more effectively at closer distances than at longer distances.
To summarize, we can guess the relative speed of an object by the quickness of change in its position on an ‘apparently’ rigid frame of reference (in the case of a train, it is the distant natural backdrop).
The problem comes when we are directly facing the oncoming train in the line of sight. The initial and final positions of the engine (object) are on the same line of sight.
There is no need for the lateral movement of our eyes. Further, the train might be too farther from us to provide us the binocular vision of the changing depth of engine’s position. In the absence of a need to move our eyes sideways because of the lack of changing positional parameter of the engine over a frame of reference lateral to us and due to poor possibility for the binocular estimation of the positional depth of the engine from us, we are unable to evaluate the speed of the on coming train when we face it in its line of motion.
