The run from Chittaurgarh Jn to Nimach constitutes the so-called "cement belt" of Rajasthan and Madhya Pradesh. Rich deposits of limestone in the hills surrounding Nimbahera mean that cement manufacturing has been taking place here for a while. Even today, there are about half a dozen major cement manufacturers here.
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This might explain where the duo vanished off to. Yesterday, en route to Mandsaur, I caught twins from Ratlam hauling a BCNA consist loaded with cement awaiting merger from a siding near Nayagaon.
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Or maybe not, since I also saw a BOXN rake loaded with coal being hauled by mixed twins from Ratlam at Nimbahera :)

A part-helpful, part-propaganda flexboard at Chittaurgarh Jn also talks about the cement belt. It speaks of the doubling work that's taking place between Chittaurgarh Jn and Nimach.
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The work itself seems to be going on fast, with non-bridge earth work over for most of the way and ballast and sleepers lying ready to be laid out for some of it. For virtually all of the route, it looks like they are utilizing the alignment of the previous MG line that linked Ajmer with Ratlam (and thence went on to connect it...
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with Indore, Khandwa, Akola, and Purna). The remnants of this route (mostly in form of ballast) can be seen just next to the current BG track, in patches where the earth work isn't done yet.
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There are some places where they have smartly chosen to blow up some local geography to optimize the route. This is most apparent near the escarpment at Nayagaon, where the plains of the Berach-Chambal basin end and the Malwa plateau begins.
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One hopes that this becomes the first step towards doubling the entire stretch from Chittaurgarh to Ratlam.