If I had to guess the gap is for future expansion!
The transition from a 1910 concrete and steel viaduct to a modern steel structure is a delicate affair. Figuring the 1910 viaduct has had 111 years to settle in its ways, where a brash young upstart steel and concrete pillar will be wiggling around for a bit. A hard tie together would cause an issue for one-or-the-other or both of them.
By leaving a gap and most likely a forgiving connection should lead to harmony in there spatial relationship. When the trackage is laid there will probably be a rail expansion joint at that location.
Here are some photos I took on Friday, January 15, 2021. As you can see the new and the old will not be hard tied together.
Or, someone measured wrong!
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