The files are plain sector-by-sector headerless dumps (the same idea as
/dev/fd0). Each sector is 256 bytes, each track contains 16 sectors. Tracks are arranged in the same order as "logical tracks" in TR-DOS: 0th track on 0th side, 0th track on 1st side, 1st track on 0th side, 1st track on 1st side, …. If remaining sectors at the end of floppy are unused, the TRD file can be less than actual floppy size. These files are supported by most ZX Spectrum emulators.