the SNCF 'clothes irons'

Still a steam train in one respect - steam is coming out of the van, presumably for heating the passenger cars.  In French the word for a caboose is fourgon, literally the oven wagon.

The first electric locomotives introduced for the post-WW2 electrification in the Est region - in Bo-Bo and Co-Co variants - with their central cab and sloping hoods led to the inevitable nickname of the Fers à Repasser.

These first day covers mark the opening on 11 May 1955 of the Valenciennes to Thionville section in the new 25 kV 50 Hz AC system, as distinct from the 1.5 kV DC used to electrify lines before WW2.