They are the bogie frames, they look like commonwealth bogies. I'm not that well up on Brill interurban baggage cars but I understand that they were of two different types. As for the freight stock, there were four open wagons, two vans and a tank wagon, all on bogies. The open wagons were of two lengths, the shorter ones were low sided, in 00 scale they could be adapted from the Ratio bolster wagon kit which looks to be about the right size. I haven't seen a photograph or drawing of the larger wagons. The two vans were unpowered versions of the motor baggage cars and the tank wagon looked very like the American tank cars of the day.