The Harris trains introduced in 1956 comprised of three car types: _M__, _T__ and BT. At the time there was first and second class travel within the metro area.
M cars were traditionally 2nd class with trailers being 1st class. The BT trailer became the 2nd class trailer and was used as the transition vehicle between the 4 car 'Block' and 3 car 'Unit' which could be detached to run smaller car sets. The BT cars had an autocoupler at one end and a solid draw bar at the other. Trailer cars in Harris sets were coupled with solid draw bars at both ends.
The BT number group was 501 - 560.
The two main types were "1st" and "2nd" series cars. Numbers 501 - 530 were "1st" series cars and 531 - 560 "2nd" series.
BT 560 featured stainles steel panels on one side. The panels were weathering tests for the new material to be used on the new suburban cars of the new trains of a future order.