Just to chime in from the Mac perspective: Realcharts are absolutely featured on our end too (thank you Mario for not just presuming!). You enable this thing inside the "Realtrack Settings" dialogue. Make sure all of the applicable options there are checked and you should be good to go.
Also, for that particular String Quartet example, your optimal choice should just be the equivalent String Quartet Supertrack, which already is all-Midi. Always a good idea to check first if your target Realtrack for this purpose has a corresponding Supertrack.

Just because you can, doesn't mean you should!
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BBox 2022 Audiophile, Mac Pro Intel, OSX 10.6.8, 800x600 (TV VGA)