mkvmerge -V
mkvmerge v0.6.0

mkvmerge -i SSA_15subtitles.mkv 
File 'SSA_15subtitles.mkv': container: Matroska
Track ID 1: video (V_MS/VFW/FOURCC, XVID)
Track ID 2: audio (A_VORBIS)
Track ID 3: subtitles (S_SSA)
Track ID 4: subtitles (S_SSA)
Track ID 5: subtitles (S_SSA)
Track ID 6: subtitles (S_SSA)
Track ID 7: subtitles (S_SSA)
Track ID 8: subtitles (S_SSA)
Track ID 9: subtitles (S_SSA)
Track ID 10: subtitles (S_SSA)
Track ID 11: subtitles (S_SSA)
Track ID 12: subtitles (S_SSA)
Track ID 13: subtitles (S_SSA)
Track ID 14: subtitles (S_SSA)
Track ID 15: subtitles (S_SSA)
Track ID 16: subtitles (S_SSA)
Track ID 17: subtitles (S_SSA)
Track ID 18: subtitles (S_SSA)

(MPlayer dev-CVS-030718-12:10-3.3.1 (C) 2000-2003 MPlayer Team)

mplayer -sid 3 SSA_15subtitles.mkv
...
mplayer -sid 18 SSA_15subtitles.mkv

Problem : No Subtitles appear