The headlight on Santa Fe No. 3751.
The headlight is a device (part of all modern types of locomotives) and it is designed to help the driver see at night. It also aids pedestrians in seeing the locomotive approaching in the day-time. There are lots of headlight types, for example steam locomotives can have their headlight(s) either on the smokebox door or up near the funnel. Diesel and Electric trains' headlight(s) are similarly positioned height-wise. The placing of headlight(s) is dependent on the manufacturer's opinion of what the optimum height/angle is needed to give the driver the best possible view. Other type identifiers include: globe type, shape, glass color etc. These too are designed to aid the driver's forward view. For example, if the color (tint) of the glass is yellow the locomotive almost certainly works in an area often visited by fog. The yellow colored light beam doesn't reflect so easily of the water vapor in the air. Multiple Units often have interchangeable headlights, that is that the forward lights can become taillights and vice versa. This is due to MUs needing to swap direction, at the end of a line for instance.