Sometimes airline passengers make trips using only regional or commuter airlines, prop planes, etc., requiring numerous connections that are usually completed via a nonstop or at least direct flight (no plane changes).
How far can one travel using only commuter-rail services, whether or not alternative Amtrak service is available between the same places? (Assume that a service is non-Amtrak, even if Amtrak operates the service on a contract, if it's not marketed or advertised or ticketed as an Amtrak service; i.e., not in the Amtrak computer.)
This includes two short and unavoidable bus-bridges (using three different services), plus one ferry (which isn't strictly necessary, but makes things more interesting by involving the Long Island RR without requiring backtracking).
You can't do this same-day ... I suspect it would take a week, given connection times, but you could do:
Boston to Providence, MBTA
Providence to New London, Greyhound
New London to New Haven, Shore Line East
New Haven to Bridgeport, Metro-North
Bridgeport to Port Jefferson, Port Jefferson Ferry
Port Jefferson to Jamaica, Long Island RR
Jamaica to New York Penn Station, MTA
New York Penn Station to Newark Penn Station, PATH
Newark to Trenton, New Jersey Transit
Trenton to Camden, New Jersey Trainsit RiverLine
Camden to Philadelphia 8th St, PATCO
Philadelphia 8th St to Suburban Station, SEPTA Market-Frankford Line
Suburban Station to Newark, Delaware, SEPTA R2
Newark, DE to Elkton, DART bus 65
Elkton to Perryville, "The Bus -- Perryville Connection" (this is the service's actual name)
Perryville to Washington, MARC
Washington to Manassas, VRE
Seventeen legs, on not-quite seventeen transit agencies. (Two NJT, but one heavy and one lighter rail; two SEPTA, but one commuter rail and one subway; and then MTA, Metro-North, and LIRR are all connected on some level.)
You could start beyond Boston, and if you started way out at the end of a green line the two MBTA trips would be very different. There may be bus systems you could pick up to further points south of Manassas, but I don't know.