http://aluigi.org/papers.htm#tdudec
As far as I know it works also with TDU2.
It covers not only the savegames but also other encrypted files.
Algorithm: XTEA
Keys:
"\x2C\x43\xEA\x64\x24\x5B\xA3\xF8\x81\xCD\x8E\x01\xAC\xBE\x26\x83" // savegame
"\x4A\x3C\xE2\x4F\x11\xC2\xBA\x80\x3A\xBD\x17\x69\xBD\x8E\x52\xF0"