Install mkvtoolnix
sudo apt-get install mkvtoolnix.
Run from terminal:
mkvextract tracks <your_mkv_video> <track_numer>:<subtitle_file.srt>
Get information about tracks.
mkvmerge -i <filename>
Using this utility you can extract any track, even audio or video.
WARNING: Do not use mkvinfo as it says things like “Track number: 2 (track ID for mkvmerge & mkvextract: 1)” which are confusing.