This photo is not very in focus because it was taken at twilight at 4x zoom, but you may see the sparks coming from the large box truck riding on the tracks. I think that's from them cutting the rails in preparation for replacing them.
Following the cutter were pieces of equipment that looked like they were pulling spikes. I didn't watch the entire procession, but I would expect that crane-equipped vehicles would then come through and move the rails out the way and place the new rails, after which the track would be aligned, re-welded, re-spiked, and the ballast cleaned and tamped.
On our way home from Tonopah, we saw the Rail Gang's equipment stored on a maintenance-of-way siding at Thsibe (that's the siding west of Fernley, accessible from the Derby Dam exit on I-80), and stopped to take a look. Click through these photos to look at the rest of the equipment. Note the mountain of spikes stacked next to this gondola. When I came by on Tuesday night, instead of spikes, there was a mound of rails instead.
Later, one of the electromagnet-equipped cranes came through to collect all of the loose ties and tie-plates. The equipment makes a sound I would associate from an SF movie of a particle beam cannon going off, but unfortunately, they never made the sound when I was out recording, and there was not enough light to see anything, anyway.