Please note the seven panels of the lift spans, as defined so well by the Max train.

The upper panels; those with diagonals, define the load-carrying truss span. The North and South diagonals define the ends of this span. The verticals next to the two towers are there to guide the lift span in it's travels. The lower (railroad) deck hangs from the load-carrying truss span.
Now, note how much heavier this span is compared to the street span in the background.

If river traffic wants to pass under, but isn't tall enough to require opening the street-level span, the operator pulls up the railroad deck by a cog drive on the verticals. If the river traffic needs more than "up to nearly streetlevel" clearance, the operator raises everything.
All Captains, world-wide, know how much vertical they need. (before we bought our sailboat, we knew the height of the wind indicator on top of our mast) We required the raising or swinging of several drawspans. Note the low-level railroad swingspan over the Swinomish Slough.

Note that the lower verticals carrying the railroad deck are slimmer than the the truss verticals that they slide up into.

My first visit to this Portland bridge, probably in the 1940s or 1950s, took a lot of looking and a lot of walking. Some river traffic helped.