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Look around the world to see what is possible with our current technology.  The Golden Gate bridge has a deck 90 feet wide (27 meters) supporting 6 lanes, and its main span is 4200 feet (1.28 km) long spanning a major shipping channel.  It's obviously plenty high and wide enough to support the large port facilities in the Bay Area.  And, it was built in 1937 (86 years ago).

Other bridges support rails.  I don't know if there is a bridge that carries 6 tracks.  That seems excessive.  Two tracks seems plenty.  That allows one train simultaneously in each direction.  Look at any chunk of 6 parallel tracks around the world.  Where are 6 tracks carrying separate trains concurrently?  Mostly you find a lot of parallel tracks in switching yards, not for carrying multiple independent trains for long distances.

It seems like with some planning, two tracks is enough.  Look around where trains are used heavily on land.  Once you get past train yards you see two tracks at most.  This includes major train routes around the country, like running east/west out of Flagstaff AZ.

So yes, what you ask seems doable with some reasonableness applied first.  And, if you can build a 2-track train bridge, you can build a wider one.  The economics don't favor doing that, though.