If the railroad catches you, you *will* be prosecuted for trespassing.
The bridge is owned by the Portland & Western Railroad, a subsidiary of Genessee & Wyoming of New York. P&W runs about 10 trains per week across that bridge shuttling railcars from the interchange with UP at the Brooklyn Yard in SE Portland. P&W's tracks join the UP tracks in north Milwaukie behind the indoor BMX joint.
Riding that bridge would be exceptionally dangerous. It's at least 70-100' above the Willamette, the trestle is at least 1/4 mile long, and it's easy to fall off. I would encourage that commuter to not use the bridge. Sure, the likelihood of a train coming across is low (unless it is late evening or early morning on a weekday), but trains are deadly and they can't stop easily. This same railroad is where a boy nearly got killed on one of the other bridges, and survived only through the heroic efforts of one of the train crew, who literally hung off the front end of the locomotive and scooped the kid up bodily just before the train hit him (the loco was moving about 10-15mph).