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".
Carowinds Monorail ended in 1994.
Carowinds Monorail was created in 1973.
Sentosa Monorail was created in 1982.
Sentosa Monorail ended in 2005.
Tokyo Monorail was created in 1964.
Kitakyūshū Monorail was created in 1985.
Okinawa Monorail was created in 2003.
Metro Monorail was created in 1988.
KL Monorail was created in 2003.
Moscow Monorail was created in 2004.
Johnny on the Monorail was created in 1980.
The base word for "monorail" is "rail."