looking at Apple Maps + thinking about the most unhinged transit trip you could take from NYC-DC, which would probs go something like this (haven’t checked schedules tho)
- MTA-PATH
- PATH to Newark
- NJT Newark-Trenton
- River Line Trenton- Camden
- PATCO Camden-8th St Philly

SEPTA MFL 8th st-30th st
SEPTA RR 30th st-Newark DE
Amtrak (only rail option here) Newark DE-Perryville MD
MARC Penn Perryville-Union Station DC
WMATA Union Station-Final Destination

OR if you wanted to get in even more systems:
Amtrak Newark DE-Perryville
MARC Penn to Baltimore Penn
Light Rail from Baltimore Penn to Camden
MARC Camden to Union Station

I know the MARC trains don’t run a lot so this is probably completely in feasible tho lol

At some point or another I def want to try some version of an NYC-Philly all-regional transit trip (mostly to cross the River Line and PATCO off my Transit Life List) but the Philly-DC leg seems less fun