In regards to India, I do believe it had the resources for a fairly late industrialization. Most Indian coal was concentrated in Jharkand, and is inaccessible until the mid-late 1800s. However, the coal reserves at Jharkand are quite massive and there'd be more than enough for a war. Vast amounts of metals in the same region could also allow for the production of quite a bit of Indian weaponry.
In addition, the Himalaya range effectively blocks out the Indians and Chinese from fighting through Tibet, so there'd only really be the place around Assam to fight one another in. That makes a war of attrition possible.
I don't think a war with 50 million soldiers on each side is possible at all with 1914 technology, however.
Yeah, they're massive, but we're presuming that to industrialise and be on the level of a great power (or even a Tsarist Russia), India has been exploiting them and the rest of its resources for a century or so. The same goes with China's own huge amount of natural resources.
There's also the whole "from a Eurocentric viewpoint economically, these resources are worthless for their time, but from a Sino/Indocentric viewpoint economically...?" What the Raj deemed inaccessible/of questionable economic value might not be what this industrial India deems inaccessible or of questionable economic value.
sure, why not? China and India can basically do this today at tech levels higher than 1914, the world could prob fight WWII - nukes 2 or 3 times
How can you even supply all those troops through the Himalayas, let alone break through? That's like the Alps front in WWI on steroids (trenches at the top of Mt. Everest, nice). The only place to attack China is from the Pacific coast (hard), the northern steppes (easiest) or the Southeast Asian jungles. Even Assam and Northern Burma aren't easy points to attack China from, since Yunnan has such a crazy mix of terrain.
Unless you have these 50 million Indian soldiers fighting elsewhere, like in Vietnam/Laos or in North Asia. That makes a bit more sense. But it's unlikely to have those 50 million man armies until one side is directly invaded, which will take a victory on a front actually conducive to invasion. Since if you feed a million men through the passes in Central Asia, you'll be lucky to get past OTL Xinjiang before the Chinese army utterly wrecks you.
My guess is you'd have 50 million men on each side raised over the course of the war, never all at once. There just isn't enough frontage to put 50 million men unless your only strategy is a cartoonish level of IRL zerg rushing. That's a comparable level to OTL France or Germany by population, since I'd assume about 600 million in both China and India since we presumably don't have mass famines/brutal rebellions, etc. like OTL China and India had leading up to 1914.
However, if the pace of resource exploration/exploitation picks up globally--if the world outside Asia's mostly a bunch of colonies, you'll need far more intense exploitation than OTL's colonies had to fill the demand--it is conceivable each side can support that many troops.