Whilst looking around as one does I found this useful site.
Little River railcarsIf you scroll down (Mikey) you will find a gasoline railcar used well before 1920.
There is alot a lot of information about model railcars and railbuses.
Also it just so happens that there is a very nice limited edition Accucraft/Delton brass railcar for sale on German e bay.
Aristo railcarApparently this is a model of a White railcar originally built in
1918.And indeed it is....
White rail busSo although we can't prove that railcars in France were used before the early 1920's they certainly existed in the USA.