very early JTT prototype, created from https://github.com/dret/JTT/blob/master/GPX/test.gpx via https://github.com/dret/JTT/blob/master/GPX/GPX2JTT/GPX2JTT.xsl

test.json

{ "JTT" : [
   { "track" : {
     "title" : "Test Activity",
     "desc" : "Simple test activity walking around the block with a heart rate monitor, stopping once, and marking one lap. Recorded with Garmin Fenix 3, uploaded to Garmin Connect, and exported as GPX and TCX. Both exports are crippled because they do not contain all data fields of the activity.",
     "segments" : [
       { "data-fields" : ["latitude", "longitude", "elevation", "temperature", "HR" ] }, 
       [
         [ 47.407614681869745, 8.553115781396627, 451.79998779296875, "2015-11-13T12:57:24.000Z", 28.0, 76],
         [ 47.40762138739228, 8.553108656778932, 451.0, "2015-11-13T12:57:25.000Z", 28.0, 76],
         [ 47.407626835629344, 8.553094072267413, 450.0, "2015-11-13T12:57:26.000Z", 28.0, 76],
         [ 47.407668409869075, 8.552960716187954, 449.0, "2015-11-13T12:57:33.000Z", 28.0, 78],
         [ 47.40770772099495, 8.552867090329528, 448.79998779296875, "2015-11-13T12:57:39.000Z", 28.0, 75],
         [ 47.407753402367234, 8.5526111908257, 448.79998779296875, "2015-11-13T12:57:54.000Z", 27.0, 75],
         [ 47.40781215950847, 8.552352441474795, 448.20001220703125, "2015-11-13T12:58:04.000Z", 27.0, 73],
         [ 47.407849207520485, 8.552198382094502, 448.20001220703125, "2015-11-13T12:58:12.000Z", 27.0, 76],
         [ 47.40790771320462, 8.552007442340255, 448.0, "2015-11-13T12:58:23.000Z", 27.0, 75],
         [ 47.40796320140362, 8.551924461498857, 447.79998779296875, "2015-11-13T12:58:29.000Z", 27.0, 78],
         [ 47.4079969804734, 8.551919935271144, 447.6000061035156, "2015-11-13T12:58:32.000Z", 27.0, 78],
         [ 47.408049115911126, 8.551959414035082, 447.20001220703125, "2015-11-13T12:58:37.000Z", 27.0, 77],
         [ 47.408086750656366, 8.552005849778652, 446.79998779296875, "2015-11-13T12:58:41.000Z", 27.0, 76],
         [ 47.40809513255954, 8.552019177004695, 446.6000061035156, "2015-11-13T12:58:42.000Z", 27.0, 73],
         [ 47.408130671828985, 8.552070977166295, 446.20001220703125, "2015-11-13T12:58:46.000Z", 27.0, 70],
         [ 47.40816361270845, 8.552097212523222, 446.0, "2015-11-13T12:58:49.000Z", 27.0, 71],
         [ 47.40818230435252, 8.552138702943921, 445.6000061035156, "2015-11-13T12:58:51.000Z", 27.0, 71],
         [ 47.40825707092881, 8.552154125645757, 445.3999938964844, "2015-11-13T12:58:58.000Z", 27.0, 70],
         [ 47.40829017944634, 8.55216016061604, 445.3999938964844, "2015-11-13T12:59:01.000Z", 27.0, 71],
         [ 47.40830333903432, 8.552153706550598, 445.20001220703125, "2015-11-13T12:59:03.000Z", 27.0, 71],
         [ 47.408308032900095, 8.552153706550598, 445.20001220703125, "2015-11-13T12:59:04.000Z", 27.0, 70],
         [ 47.40834197960794, 8.552186479791999, 445.3999938964844, "2015-11-13T13:01:07.000Z", 27.0, 85],
         [ 47.40833795629442, 8.552188323810697, 445.3999938964844, "2015-11-13T13:01:08.000Z", 27.0, 84],
         [ 47.40833401679993, 8.552198549732566, 445.3999938964844, "2015-11-13T13:01:09.000Z", 27.0, 84],
         [ 47.40832010284066, 8.552253786474466, 445.3999938964844, "2015-11-13T13:01:12.000Z", 27.0, 84],
         [ 47.40828674286604, 8.552344981580973, 445.0, "2015-11-13T13:01:17.000Z", 27.0, 80],
         [ 47.40825455635786, 8.552448330447078, 444.79998779296875, "2015-11-13T13:01:23.000Z", 27.0, 79],
         [ 47.40825262852013, 8.552489904686809, 444.79998779296875, "2015-11-13T13:01:26.000Z", 27.0, 80],
         [ 47.40821675397456, 8.55262778699398, 444.79998779296875, "2015-11-13T13:01:35.000Z", 27.0, 79],
         [ 47.40820610895753, 8.552680592983961, 444.6000061035156, "2015-11-13T13:01:38.000Z", 27.0, 79],
         [ 47.40816168487072, 8.552955100312829, 444.6000061035156, "2015-11-13T13:01:52.000Z", 27.0, 76],
         [ 47.408138466998935, 8.553202534094453, 444.20001220703125, "2015-11-13T13:02:03.000Z", 27.0, 76],
         [ 47.408125810325146, 8.553239582106471, 444.0, "2015-11-13T13:02:05.000Z", 27.0, 75],
         [ 47.40811625495553, 8.553247042000294, 443.79998779296875, "2015-11-13T13:02:06.000Z", 27.0, 75],
         [ 47.40810167044401, 8.553252154961228, 443.6000061035156, "2015-11-13T13:02:07.000Z", 27.0, 76],
         [ 47.40808733738959, 8.553255340084434, 443.6000061035156, "2015-11-13T13:02:08.000Z", 27.0, 77],
         [ 47.408021707087755, 8.553288113325834, 444.20001220703125, "2015-11-13T13:02:13.000Z", 27.0, 80],
         [ 47.4079795461148, 8.55328886769712, 444.6000061035156, "2015-11-13T13:02:17.000Z", 27.0, 82],
         [ 47.40796043537557, 8.553284592926502, 444.79998779296875, "2015-11-13T13:02:19.000Z", 27.0, 83],
         [ 47.40791064687073, 8.553258189931512, 445.20001220703125, "2015-11-13T13:02:23.000Z", 26.0, 86],
         [ 47.40785448811948, 8.553215442225337, 445.6000061035156, "2015-11-13T13:02:27.000Z", 26.0, 89],
         [ 47.40780486725271, 8.553174622356892, 446.20001220703125, "2015-11-13T13:02:32.000Z", 26.0, 92],
         [ 47.40777527913451, 8.553162636235356, 446.79998779296875, "2015-11-13T13:02:35.000Z", 26.0, 94],
         [ 47.40773663856089, 8.553122906014323, 447.3999938964844, "2015-11-13T13:02:39.000Z", 26.0, 96],
         [ 47.40771467797458, 8.553109411150217, 447.6000061035156, "2015-11-13T13:02:41.000Z", 26.0, 98],
         [ 47.407703530043364, 8.55310220271349, 448.0, "2015-11-13T13:02:42.000Z", 26.0, 98],
         [ 47.407659692689776, 8.553062472492456, 448.79998779296875, "2015-11-13T13:02:46.000Z", 26.0, 101],
         [ 47.40764569491148, 8.553053420037031, 449.0, "2015-11-13T13:02:47.000Z", 26.0, 102],
         [ 47.40763236768544, 8.553045960143209, 449.20001220703125, "2015-11-13T13:02:48.000Z", 26.0, 103]
       ]
     ]
   } }
] }

hard to tell @tobixen, since there is no format definition to validate against. looking at this 6 years later i definitely wouldn't create this "spreadsheet-like" structure with "heading row" with field names and then all "data rows" following that structure. that seems to invite a lot of brittleness.