I did this a few years back 2015 the train arrives at 4 or 5 am so unless you spend a lot of time with bike assembly, traffic won’t be an issue. If I remember right I rode a street about 1 block west of the most direct route (cherry way or something) the most direct route is to simply go on the same street Amtrak is on to the trailhead, it’s only a few blocks you could even walk it on the sidewalk if needed. What traffic I encountered was very civilized. Guess they are used to seeing bikes. Same thing in DC if you are going that far, lots of bike infrastructure and no problem with traffic.
In Pittsburg I then rode the trail on the north side of the river and crossed on the Hot Metal Bridge to the south side. This assumes you are coming from west the train from east arrives at different time. After you arrive at the trailhead in Pittsburg the trail is complete to Cumberland.