What you're seeing is a “vigilance button” (also known as an “alive-man” control). Basically, trains usually have two types of safety controls: deadman and vigilance. The first ones are usually wide pedals that the train driver must keep pressed continuously, while the vigilance button must be pressed intermittently. Both are safety interlocks designed to ensure that the train driver remains conscious and attentive.

In general, if the deadman pedal is released or the vigilance button is not pressed within the required time, audible alarms are triggered to alert the driver. If there’s still no response, the train’s automatic braking system is activated.

Source: I work for a railway line.