Final answer:
When crossing railroad tracks, it's imperative to not change gears for safety reasons, ensuring the vehicle does not stall on the tracks. The Doppler effect explains the shift in pitch of a train's horn, which can be matched by adjusting vehicle speed. Standing too close to fast-moving trains is dangerous due to sudden drops in atmospheric pressure.
Step-by-step explanation:
When crossing railroad tracks, the correct procedure is to not change gears. This is for safety reasons as changing gears can result in loss of momentum or stalling, making a vehicle more vulnerable on the tracks. Furthermore, crossing tracks should be done with caution to ensure that there is enough clearance for the vehicle and without displacing the tracks, which is generally not a concern with standard vehicles and well-maintained tracks.
In the context of the questions provided, the sound of a train's horn heard at a lower pitch is a phenomenon described by the Doppler effect. To match the speed of the train and experience a pitch that corresponds to the train's actual frequency, one would need to adjust their speed. If the pitch is lower, it indicates that the relative velocity between the observer and the source is increasing, so one would either increase their speed if they are trailing the train or decrease it if moving in the same direction but at a faster rate.
The danger of standing close to railroad tracks when a rapidly moving commuter train passes is due to the rapid displacement of air creating a low-pressure area. When a train speeds past, it displaces air around it, resulting in a sudden drop in atmospheric pressure that can cause a person to be pulled towards the train.