I used to work the midnight hostler job at Alliance a few years ago. Our work was mainly moving locomotives between the roundhouse and IMF for the Q and Z trains, turning the consists when needed, etc. A couple times a week we'd have to pull and spot GE. When our work was done we'd grab whatever power was handy and head out to the GE lead to pick up the locomotives they'd dropped off. We would take the new locomotives back to the yard and position them for whatever outbound train they were scheduled on. With the GE lead cleared out of new locomotives we would then gather up the rebuild cores, air test them then drag them to the lead to spot them and come back light power. The two round trips would take a couple hours or more if the consist had to be put together or if something went wrong with the air test. Then of course you aren't getting out there if any traffic is coming so it all depended on if there was a window to do the work.
Before the new Main 1 was built in 2015 the old Main 2 (original GC&SF Main track) passed right by the GE plant. You can still see it from TX 114 to just south of Justin along FM 156. It is used today by GE to test the new locomotives (it's a great place to get photos of these locomotives, by the way). A second track was constructed parallel to it for storage a few years ago as the business model shifted from new construction to rebuilding older locomotives.
The day the GE plant tracks were cut in to the main (December 2012 maybe?) I was called to bring a locomotive to the plant to conduct testing of the tracks and certify them for use. The following day would have a grand opening celebration and open house but this day was relatively quiet by comparison. For a couple years after the plant opened we would go in to collect the new locomotives until the original Main was taken out of service after the new Main 1 was cut in. Now we only reach into the GE lead just up to the derail near the signal at Lambert.
