I have often heard that the Great War (WWI) was where the widespread adoption of specialised weapons (e.g. hand grenades, light machine guns) and roles were introduced at the platoon level was introduced for the first time. For instance, a standard British platoon in 1917 was divided into four sections (a machine gun section, a grenade throwing section, a rifle grenade section, and a rifle/sharpshooter section). Prior to that, platoons were largely organised such that every member was a rifleman and the specialised weapons were organised at higher levels; with the company being the basic unit instead.
With that in mind, my question is this:
How widespread was the training of the specialised weapons/roles - be it formally or informally? Was every soldier given a degree of training in operating things like the light machine gun, grenades and trench mortars irrespective of their role in the platoon or was it just reserved to the soldiers who would be expected to operate them? If the latter, how did the military account for attrition? For instance, the loss of a gunner in a machine gun section would severely impact the efficacy of the platoon in contrast to the loss of an ammo carrier in the same section.
From the phrasing of the question, my interest is primarily around the British Army (which I think has more accessible documentation) though I'd be interested in all factions from the war. Thanks in advance.
So here's a good starting point:
https://www.bl.uk/world-war-one/articles/training-to-be-a-soldier
It's very broad, but it should help answer your questions. Essentially, specialist schools were established and men would be sent to them after basic training. Basic training would consist mostly of military drill, physical training, establishing obedience to orders, placing the recruits under stress to acclimate them, and then very limited weapons training, combat drills, route marches, and so on. From there, they would then leave for their respective schools and become proficient with their jobs and sent off to their unit. Once at their unit, they would receivce "on the job" training which can vary wildly, but is both formal and informal. Formal being training exercises from division-level down individual action training (marksmanship on the range, bounding, etc.) refresher courses, and operational rehearsals where they would practice planned attacks on mock ups based on an upcoming objective. Informal training would be learning things from more experienced soldiers or ones on the line with you, such as how to operate machine guns, mortars, and the like if the need arose.
Providing replacements for combat losses varied. If possible, they would rotate the unit off the line, and fresh troops would be added in bulk and then they would have a chance to train together and reestablish unit cohesion, with the side benefit of providing a break to the veterans. Other times, fresh troops would be sent to a unit directly on the line. And still other times, they just made do with none and filled up their table of organization with who they had, merging smaller tactical elements together.
For replacing specialties in combat, if you're an ammo carrier for a machine gunner on the table of organization, you were a machine gunner by specialty, just a junior one. Machine guns are generally called "crew served weapons" with the exception of light machine guns designed to be operated by one man but still benefit from others. A machine gun platoon or squad were men who all went to a school to be a machine gunner, however you didn't need that many men specifically pulling the trigger on one. You had men who would carry spare ammo and a rifle to hold security on the flanks and rear and if the enemy got close. You had the team/squad/platoon leaders who would coordinate machine gun fire to achieve their objective while controlling the rates of fire to prevent over-consumption of ammo and over heating. You'd have the assistant gunner, who would often be the man carrying the tripod, bipod, and spare barrel if applicable and would also serve as a second pair of eyes for the gunner, giving adjustments to fire, reloading with the gunner, laying partially on top of the gunner to help control recoil, and helping to clear stoppages. However, if needed, you could bump those men "up" to gunners as needed and replace the ammo carriers with other infantrymen. Too many losses, and you would need to get replacements from the rear, officially. Unofficially, when you're on the line, you train everyone on how to do each other's jobs just in case. That was one of the major changes to training and doctrine that occurred, along with the rise of modern day small unit tactics that began with infiltration tactics or stormtrooper tactics, the pushing of tactical leadership and initiative taking to the lowest ranks, and more intense and lengthier training regimens and emphasis on flexibility so every man knew how to do each other's jobs and the job of the man two levels higher than him.
There are lots of really great books on the subject and I can go on about the tactical changes in depth for infantrymen over the 20th century. It's really fascinating seeing how, even right from the get go, everyone in WW1 saw that they needed to change how they did things, but they just couldn't get it to work right for years.