Marx Trains released these in 5571 bridges in 1997. The model number 5571 would be followed by -8 for 8inch or -12 for 12 inch. Truss under support versions would have a U at the end of the number, 5571-8, 5571-8U etc. Some flat cars came with the 8 inch bridge side as a load.
The 5584 was released a year later and as far as I know, was always 8 inch and truss under support. Truss under support bridges came with a mounting kit.
