1090 is the NEC, 3 coach "add on section" of 90. ItThey are showing as separate trains because of the track work along the route. If 90 is operating late enough coming to PTB, they will run 1090 independently of 90. Today is an example of when they will use 1090 since 90 is delayed due to a grade crossing incident. When the actual 90 shows up in DC, it will operate as a discharge train only.
If it is reasonably close to on time, they will put the local coaches on 90 and operate as one train.