There have been two distinct swing door carriage classes coded BT.
From 1919 to 1923, steam hauled Second Class class _B__ cars were converted to electric service and recoded BT with new numbers.
The BT numbers for this group were: 1 - 74, 76 - 85, 87, 94, 96, 100. There were eighty-eight cars in this group.
Between 1922 and 1925, most of the BT group were recoded to _T__.
BT 1 - 74 became T 27 - 100, BT 76 - 85 became T 102 - 111, BT 100 became T 126.
Cars BT 87, 94 and 96 were converted back to steam hauled stock as B cars, the original car numbers they were converted from: BT 87 to B 54, BT 94 to B 63, BT 96 to B 66. It seems likely that these cars were surplus to the electric fleet once the project was abandoned in 1923 and no more swing door _M__ cars were converted.
To provide for more Second Class cars, twenty T cars were recoded to BT during 1940/1941. The original T number was retained. This meant that for most of the cars, this was the second use of BT but with a different number. For example BT 41 ex T 41 ex BT 15.
Six more cars were converted from _T__ to BT in 1957, two in 1963, six in 1965, one in 1968.
The numbers for this group were: 4, 7, 8, 12, 14, 18, 25, 29, 36, 38, 41, 42, 45, 47, 49 - 52, 54, 62, 64, 69, 76, 77, 79, 85, 87, 96, 102.
The group was scrapped progressively from 1962 to 1973. Some cars were converted to _WW_, Way and Works sleeping cars.