I regularly see work trains go by my house on the Blue line late on Sunday nights. They're usually two flatbeds with out of service 2 car combos at either end (2200 or 2400's I believe, but they may not be the same cars all the time).
What is on the flatbeds changes most weekends. While the Blue Line was shut down on weekends I would regularly see one with a small crane on it, while the 2nd had ties and other railbed components. There's also one with a gigantic plastic tank full of what I assume to be a cleaning solution. Living next to the El has given me a much better appreciation for how much maintenance is done to the tracks. Almost every day I see crews walking the track checking the rails and every weekend (and most nights) there are work trains going by.