This is not a word which would really have an antonym. There is no special word for a railway with two rails - it is called a "conventional railway".
The base word for "monorail" is "rail."
Johnny on the Monorail was created in 1980.
Sentosa Monorail was created in 1982.
Sentosa Monorail ended in 2005.
Carowinds Monorail ended in 1994.
Carowinds Monorail was created in 1973.
Okinawa Monorail was created in 2003.
Kitakyūshū Monorail was created in 1985.
Metro Monorail was created in 1988.
Moscow Monorail was created in 2004.
KL Monorail was created in 2003.
Tokyo Monorail was created in 1964.