The current Steel_Bridge opened in 1912 with tremendous capacity well over a century later. Perhaps the designers looked at the narrow New Westminster Rail Bridge & decided to avoid the BC bottleneck approach to things.
Upper: 2 outer lanes for general traffic, 2 inner lanes solely for MAX Light Rail, and sidewalks on both sides
Lower: Union Pacific Railroad (incl. Amtrak toward Eugene) and walkway.
https://en.wikipedia.org/wiki/Steel_Bridge#History - https://www.youtube.com/watch?v=r02EbmjuNfw
https://en.wikipedia.org/wiki/New_Westminster_Bridge Opened in 1904 and like so much infrastructure in BC, it wasn't built for any significant future capacity.
http://www.gvgc.ca/v_Rail.aspx
https://www.tpsgc-pwgsc.gc.ca/biens-property/construction/new-westminster-eng.html
https://thetyee.ca/News/2009/06/01/RailFix
Unlike Portland, NW never seemed to ever want to become a big bustling river city, just another provincial backwater. There was a time in the 1800s when NW could have acquired what would eventually become known as the Tri-Cities. However, that wouldn't fit within its backwater BC mentality.
Indeed, to this day, the former BC capital & Victoria are quite small when compared to Edmonton, Regina, Winnipeg & Q. City. However, since little NW is in the middle of Greater_Vancouver, it has been gradually encouraged to take on more big city attributes.
