ffmpeg -i two-text-tracks.mkv -map 0:s:1 -c:s copy captions-es.srt
exit=0
Input #0, matroska,webm, from 'two-text-tracks.mkv':
  Metadata:
    COMPATIBLE_BRANDS: isomiso2avc1mp41
    MAJOR_BRAND     : isom
    MINOR_VERSION   : 512
    ENCODER         : Lavf61.7.100
  Duration: 00:00:04.00, start: 0.000000, bitrate: 35 kb/s
  Stream #0:0: Video: h264 (High), yuv420p(progressive), 960x540, 24 fps, 24 tbr, 1k tbn (default)
      Metadata:
        HANDLER_NAME    : VideoHandler
        VENDOR_ID       : [0][0][0][0]
        ENCODER         : Lavc61.19.100 libx264
        DURATION        : 00:00:04.000000000
  Stream #0:1(eng): Subtitle: subrip (srt) (default)
      Metadata:
        DURATION        : 00:00:03.125000000
  Stream #0:2(spa): Subtitle: subrip (srt)
      Metadata:
        DURATION        : 00:00:03.500000000
Stream mapping:
  Stream #0:2 -> #0:0 (copy)
Output #0, srt, to 'captions-es.srt':
  Metadata:
    COMPATIBLE_BRANDS: isomiso2avc1mp41
    MAJOR_BRAND     : isom
    MINOR_VERSION   : 512
    encoder         : Lavf61.7.100
  Stream #0:0(spa): Subtitle: subrip
      Metadata:
        DURATION        : 00:00:03.500000000
[out#0/srt @ 00000250cd9ed8c0] video:0KiB audio:0KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 89.473684%
size=       0KiB time=00:00:03.50 bitrate=   0.3kbits/s speed= 517x