Extracting subtitles with MKVmerge

check the tracks with:

mkvinfo "filename.mkv"

extract the subtitle track with:

mkvextract tracks "filename.mkv" 3:subs.srt

Cutting MKV files with 'mkvmerge'

mkvmerge --split timecodes:00:28:00,00:34:42 'input.mkv' -o 'output.mkv'