That's a commonly reported bug. The Quark scenery is the most complicated one - there are computer-driven trains, cars and automatic railway signals. All this causes that this scenery doesn't run smoothly.
There is a 'trick" that will enhance performance in the Quark scenery a bit. Open the quarkmce.scn file in Notepad, search for the word "przeszkadzajki". It should be at the end of the file, right after the trainset strings. Now, delete everything after the word "przeszkadzajki". After this your file should end with 'endtrainset' statement. If you're unsure, backup the original quarkmce.scn file.