The usual. Traffic with CSX in regions that have more than enough room to add another track, but neither side wants to pay for it. Not to mention, there's no express version of the Capital Limited that only stops at major cities, so you also have to stop for small towns in rural MD and OH which take some time. Furthermore, the fallen flag counterparts of almost every Amtrak train that's limited to 79 due to PTC went at least 90 or 100 at one point (look at the Milwaukee Road!). Upgrades in tracks and signaling are therefore necessary to increase speeds from the federally mandated 79; upgrades which can best be paid for by companies with more funding to do so (such as Brightline and FEC).
"Look down, step over, and watch the gap!" - Dr. John Clarke, The Gap Rap (2010)