There was a more frequent service in place in the 1970s and 1980s (5-6 trains per day each way - see Table 32 of the 1979, 1985 & 1988 National Passenger timetables for more details), but the timetable was cut in half in 1991 due to a rolling stock shortage and to the present minimal level in 2004.
The frequency of train services increased during the 1970s and 1980s, then decreased significantly in 1991 due to a shortage of rolling stock, resulting in a current low level of service in 2004.