By the late 1980's, passenger traffic had increased leading to congestion on the three car train sets planned in the early 1980's. The interurban sets were expanded to four cars with the addition of another car.
These new cars were coded BIH. The author presumes the ' I ' indicates 'Intermediate'.
BIH cars were issued to service between 1989 and 1990. Twelve cars were converted. Car numbers were 181 - 186, 189 - 194.
Cars 181 to 186 were converted from Harris _T__ trailer cars.
Cars 189 to 194 were converted from the 'Grey Ghost' Harris trailers rebuilt by Comeng at Dandenong.