Locomotives
There were many types of locomotives used by all sides during WW1. The WDLR (including British, Canadian & Australians) system employed examples from Hudson, Hunslet, Andrew Barclay, Baldwin, ALCO, Baguley, Motor Rail, Dick Kerr & Naysmyth Wilson.
War is the mother of invention, so the saying goes - this was indeed the case during WW1 - this was the first time that internal combustion locomotives & bogie rolling stock were used in any quantity. The level of standardization of rolling stock was unprecedented at the time.
British (WDLR)
The British locomotive roster boasted locomotives from 9 different suppliers. These locomotives were of two main types, Steam & Internal Combustion.
The British forces used several different types of Steam locomotive - these were used in rear areas for moving large amounts of supplies to forward marshalling yards, where the less obtrusive Petrol Tractors too over - working the trains, in some cases, right to the front.
The WDLR operated 4-6-0T locomotives from the British manufacturer Hunslet and their far more numerous Baldwin counter parts. Also operated were 2-6-2T ALCO, 0-6-0 Hudson & Andrew Barclay well tank locos.
Baldwin 4-6-0T Steam locomotive.
Prior to the start of WW1, France was developing an extensive military railway system. As part of this a standard range of locomotives & wagons were designed. These mainly consisted of Pechot-Bourdon 0-4-4-0T's, Decauville 0-6-0T's and Crochat/Schneider tractors. On the outset of WW1, France's manufacturers were unable to cope with demand, thus they placed orders with foreign builders. In Britain Kerr Stuart & Co. were building Decauville type 0-6-0T's, their 'Joffre' class.
French Artillery Railways 'Joffre' in action (unknown location) c1917 - Photo: Georges Mangin/Didier Oberlin
North British, in Scotland were contracted to build some Pechot-Bourdon 0-4-4-0T type locos under license, the eventual number of these built is unclear. In the USA, the famous firm of Baldwin, under took to build 280 of these machines in addition to some unusual 0-6-0ST locos.
French Artillery Railways Baldwins B-15 & B-6 (unknown locations) c1917 - Photo: Georges Mangin/Didier Oberlin

These distinctive locomotives were designed with the following criteria in mind;
(i) a 3.5 tonne axle load; (ii) keeping it light enough to be transported empty on a 10-tonne mainline wagon.

(Thanks to Dave Sherron)
American
When the Americans entered the conflict, they were able to learn from mistakes made by the other allied forces when designing their own equipment. The steam locomotives were supplied to the same broad design (2-6-2T, very similar to the British ALCO's) from 3 main suppliers, Vulcan, Baldwin & Davenport.
American Light Railway - new Baldwin 2-6-2T locos delivered to France (unknown location) c1917 - Photo: Georges Mangin/Didier Oberlin
By the time these engines were being delivered en-masse, the war was drawing to an end & many of them were diverted to US Army bases for training purposes. The Baldwin locomotives are the only ones known to have seen active service in any number.
When it came to the internal combustion locomotives, there was already a suitable Baldwin loco design in use with the French - the 50hp Tractor. This was a 'Maid of All Work' and could be found doing most tasks. Further information can be found on JR Welsh's interesting 'Critter' article Click here or click on image.
As a development of this engine a smaller lighter 35hp machine was produced, primarily for forestry use.
As further pictures and drawings become available I will endeavor to further enhance this area. Please contact me at the following e-mail address (to prevent SPAM you will need to replace (AT) with @ to make the address work!) gareth(AT)wdlr.org.uk if you would like to contribute.