Landships II

They may have had coil springs on the inside. In Railway Artillery Vol 2, Harry W Miller says of the 400mm howitzer's rail trucks (bogies):

There is nothing unusual in the design of the truck. Attention is called to the fact that the journals of these axles are on the inside of these wheels and in consequence the journal bearings are most difficult to replace.

You'll have noticed that the 400's bogies had wheels completely external to the bogie body, not very common in overall bogie design history as far as I can make out, but common to several French, and US, railway gun mountings of the period (see attached). They most resemble the image of a British locomotive bogie I've attached, which sports coil-springs inside the bogie frames. Perhaps the French bogie was similar?

EDIT: Doing a search I noticed that I was involved in a discussion here in 2005 which touched upon whether or not this type of French bogie had suspension. To be brutally frank, I am none the wiser now than I was then...

Nothing to do with answering your specific query, you may nevertheless be interested in this:

http://www.ecpad.fr/des-pieces-dartillerie-lourde-sur-voie-ferree-alvf-dans-la-marne

Click 'Voir la galerie photo' for some splendid images. If you then allow your cursor to hover over each image and right-click 'View Image' it opens a really large version of the photo.