I am seriously considering something with sed, awk and vi...
if it's a one-off split into 5 files, use vi.
this is based on a track gpx from cycle.travel, that has 1 line per <trkpt>, 8 lines above the first <trkpt>, and 3 lines after the last <trkpt>
:1 go to first line
"a8yy copy 8 lines into buffer a (or however many lines it is above the first <trkpt>)
G go to the end
2k go up 2 lines, to <trkseg>, after the last <trkpt>
"b3yy copy the last 3 lines into buffer b
:1 go back to the top
change the name of the track to add " section 1"
moving to the name item will be click on it, using the arrow keys, or using the hjkl keys (h=left, j=down, k=up, l=left), depending on the version of vi you are using.
When the cursor is in the right place, go into edit mode by either "i" (insert before), "a" (append after) or "R" (replace/overtype), type the addition to the name, then use the <esc> key to get back to command mode.
:8 go to line 8 (line above the first <trkpt>)
3000j go down 3000 <trkpt>s (if a <trkpt> </trkpt> pair occupy several lines, multiply 3000 by the value of several)
"bp paste buffer b below the 3000th <trkpt>
2j go down 2 lines to the </gpx> line
:1,.w section1.gpx write out the first segment to file section1.gpx
:1,.d delete the first segment
"aP paste buffer a above the <trkpt> the delete left you on
then change the name of the track in the pasted header section, and repeat from ":8", until "3000j" fails because there aren't that many points left, at which point write out what's left by ...
:1,$ w sectionN.gpx
:q! quit without saving, so you've still got the 15000 line version
I mostly use
WinVi, as being a simple executable that doesn't need an install