- A bogie is set of 4 wheels below the coach and not the coach itself. The correct word is “coach” or “train car”.
The big ‘X’ on the last coach is an end of (train) rake marker. There is also a smaller red plate with white letters “LV” on the car.
This marker (and plate) means “last vehicle”. This indicates to the station controller at each station that the entire train has passed the section, no coach / car has been left behind due to any accidental decoupling in between the rake. Every station controller, especially where the train does not stop, keep a check on this and alerts the station master, loco driver and previous stations that a section of the train is missing.
Edit: Do note that new LHB rakes have a different livery coach at the end of the rake like this:
While the above coach does have the X mark, new/latest rakes are doing away with this traditional approach. Instead, there is a blinking red beacon which can be seen during day or night. If you see this video (👇), you will see that the last coach doesn’t have the X mark.
The VB trains don’t need to have the mark. Depending upon the direction, the last coach has two blinking red beacons on each side. See below: