He had no truck with these

Why do some folk like rail-mounted guns so much? They pop up in films such as Doctor Zhivago - the best version, the one with Julie Christie, Tom Courtenay and Omar Sharif in which the armoured train bristling with guns is positively a character in its own right. The guns were effective fund-raisers, appearing on war-time campaign postcards, bought by the thousand by the public. They pop up in novels - the sort I wouldn't admit reading! They were loved by newspapers. Pictures from the Boer War show rail-mounted machine guns heading up a train. They are depicted on the front of serious histories. You can't get a more serious historian than Christian Wolmar and his 'Engines of War - how wars were won and lost on the railways' On the book jacket there is a picture of the massive gun the Germans used on the Russian front.
Even my 'other half', normally a reasonable person, is keen. He longs to build a model of such a gun. Oh well, go on, let's have a picture!

Postcard of rail-mounted gun sold to as part of the War Bond Campaign during World War One. The landscape suggests that the gun is on trial in Scotland. The material was passed by the censor who may have demanded that details were omitted. Messrs A.M. David London

When we were planning illustrations for our book 'Colonel Pechot:  tracks to the trenches', Malcolm was very keen to include a picture of a rail-mounted gun.   He was overjoyed when Dr Christian Cenac gave us permission to include a drawing of an affut-truck, the version of the rail-mounted gun which was used on the Pechot system. That is why 'truck' is in the heading. There are some excellent drawings in Cenac's book '60 cm track used to supply the French fronts during the 1914-18 War'
Speaking personally, I am not so keen on the affut truck/rail mounted gun. Truck=bogie can easily be confused with truc=thingy/widget, another splendid French word and so I think of them as 'rail-mounted thingymebobs'. 
To be honest, they were not a great deal of use. If you ask me, a rail-mounted gun is great for selling books but a bit explosive - like sex really.
But as you are reading on, I assume that you want to know all about the affut-truck Peigne. As the name suggests, the idea came to General Peigne. He saw the Pechot system being installed around the frontier fortresses in the 1890s. Rather than trains loading and unloading guns, why not fire them from the train!  There was even the suitable Canet gun carriage. Nothing to his mind seemed more logical and simple. Why hadn't Pechot thought of it?
The 120mm and 155 Court (short-barrelled) guns were both tried on the affut truck. http://fortificationetmemoire.fr/wp-content/uploads/2012/10/AFT2.bmpTo prevent damage to the track, there were fold-out supports which the crew deployed before shooting. Trials were not a great success. The system was a bit flimsy and the operators tended to lose the keys provided for winding out the supports.  A second generation of affut-truck had more sturdy fold-out supports. The fiddly keys were replaced with solid bars with special cradles on the supports.
A photographs in Voie Etroite magazine no 81 shows  an affut-truck with 18 soldiers, plus an inspection squad.  Photographs taken around the place forte/frontier fortress of Epinal  circa 1900 suggest that each gun had a crew of five, not counting the crews bringing ammunition. http://www.fortiffsere.fr/artillerie/index_fichiers/Page1488.htm
Exercises showed that these guns were not actually much use. If they fired, they gave away their position to observers.They could move to shelter before enemy artillery were trained on them, but the rail could not and it suffered accordingly. No wonder Pechot did not approve!
The war service of the rail-mounted gun was short - on the Pechot lines at least. Rather surprisingly, the affut-truck Peigne emerged after the war. Voie Etroite no 79 shows a German 0-8-0 Brigadelok (ceded to France as part of war reparations) pushing a rake of them along the network at Toul. But no, none have been found decaying on a forgotten siding in postwar years.