All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
A Mahram should be male, since it is meant to help, guard and maintain a woman on her journey. So, a woman or a male who is not pubescent cannot very well undertake the duties of the pubescent male.
Some scholars legalize a woman's traveling to perform the obligatory Hajj and ‘Umrah if there is a good and reliable company of women provided that the woman herself is pious and religious.
As for other kinds of travel, no woman is permitted to travel without a Mahram as stated above.
For more benefit on the prohibition for a woman to travel without a Mahram, please refer to Fataawa 262621, 88562, 193077, 102095, and 85945.
Allah knows best.