UK train driver selection uses timed aptitude tests of concentration, reaction, visual search, memory, rules, and judgement. These tests practise those task formats so assessment day is not the first time you sit this kind of work, under a clock.
Click when the circle turns green. Do nothing when it turns red.
Why it is useful
You need to act quickly when you should, and hold still when you should not. Being slow on a real go is a problem. Clicking every flash is also a problem. Watch false starts as well as your best time.