Rebuilding and digitalizing a brass gasoline rail coach

[modelsof1900]

After a year of stopping my gasoline rail coach project (it misses only adding windows, inner lighting, interior and safety chains to trucks) I found a part description of the marked part which I not could identify before.



The Car Builders’ Dictionary of 1922 indicates this part as “Back-up lamp” - and I would like to install a diode inside for lighting this lamp.
However what is this for a lamp?
Is this a red light indicating the train end while the rail coach will run backward? Or is this a white lamp – and than I do not know the use. The front lamp is mounted on roof below of the bell. So I do not think that this will be a second forward running lamp.
I good time ago I have seen an other model of this rail coach and there was a white light installed to this lamp – and I was confused for this lighting.
Please can give me anyone of you a short explanation for my understanding problem of this American term “Back-up lamp”? Thank you very much for your help.