The freight cars are American. They were built in the USA and shipped across the Atlantic in kits, then assembled in La Rochelle (France) by the US Army’s 35th Engineers. The Trains & Traction association recently got a funding grant to restore them. They are now authentic historic artifacts preserved and listed on the official French Historic Landmarks list.
The gun is French. It is a 155mm GPF designed during WWI, built in France and adopted by the US Army. It is also an authentic historic artifact.
The soldiers are also authentic-looking. They represent French soldiers of the WWI. Contrary to the cars and gun, these 4 soldiers do not date back from WWI and (unfortunately?) they are not preserved as historic artifacts!
This historic “Centennial Train” ran for a few days on the “Train des Mouettes” tourist railroad located near Royan, with an ex-Heavy Artillery on Railroad Track (ALVF) steam engine on the point, the 140C 38.
The text on the left (on the flat car) reads: American car - Return to one of the stations of Saint-Nazaire, Bassens (Bordeaux), La Rochelle, La Pallice (port of La Rochelle), Brest. They are the French ports where Army supplies were arriving straight from the USA.