.trd file is a raw dump of TR-DOS (ZX-Spectrum) floppy. .trd files are headerless and contain consequent "logical tracks", each logical track consists of 16 256-byte sectors.
Logical tracks are defined the same way as used by TR-DOS: for single-side floppies it's just a physical track number, for two-side floppies sides are interleaved, i.e. logical_track_num = (physical_track_num << 1) | side
So, this format definition is more for TR-DOS filesystem than for .trd files, which are formatless.
Strings (file names, disk label, disk password) are padded with spaces and use ZX Spectrum character set, including UDGs, block drawing chars and Basic tokens. ASCII range is mostly standard ASCII, with few characters (^, `, DEL) replaced with (up arrow, pound, copyright symbol).
.trd file can be smaller than actual floppy disk, if last logical tracks are empty (contain no file data) they can be omitted.
This page hosts a formal specification of TR-DOS flat-file disk image using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.