3 rail bridge rail is the next step up from 2 rail. In addition to the 2 - 6 X 6 box beam runs that 2 rail bridges have there is a run of 5 X 3 X 1/4 box beam at the bottom.
This picture shows the transition of a 3 rail bridge to guide rail. The bottom two rails of the bridge will terminate while the top rail attaches straight to the guide rail.
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